This is a very wide question that has many answeres depending on the parameters. The answer may be anything from 30 seconds to 60 minutes, depending on many, many factors.

• What is the artillery doctrine of the guns firing? Different countries had different doctrines, not only from each other, but some switched or modified theirs during the war.

• What is the equipment? Some guns were much quicker and easier to deploy than others. Bofors, for example, produced gun carriages that allowed the gun to be placed un uneven terrain, which of course would reduce how long time it would take for the gun crew to find a suitable place to unlimber theit howitzer.

• Where is the ammunition? Different countries had differing doctrines on where the artillery ammunition was transported, and for some it could take longer for a decent supply of shells to be available to the gun. Keeping large amounts of shells close to a gun could be very dangerous in case of counter-battery fire, but was of course convenient if you wanted to fire extended barrages or get firing quickly.

• Where is the forward observer. Is it a forward observer calling in the artillery strike? Artillery units had forward observers that went out to the unit requesting support to observe where the shells landed and correct the fire by reporting in (ie, "400 meters short, 200 meters to the right, correct and fire again"). If the forward observers are with the artillery unit, they need to get out to the unit requesting support, set up an observation point (that needed to be well protected and often concealed - artillery observers meant enemy artillery fire, thus they were prioritised targets for snipers and sharpshooters).

• What are the communications doctrine and equipment? Do the unit requesting fire support go to higher command first, or the higher unit's artillery commander (many countries had an artillery commander who decided the priority of requests and needs and ordered the artillery to fire along those priorities)? Do they send runners (or riders, mororcyclists or bicyclists) with written messages, do they roll out telegraph or phone lines for the forward observers and higher command to communicate or do they use radios?

• Do the artillery unit have pre-calculated artillery data, or do they calculate it on the spot? Do they have mechanical or analogue machines or other tools to help them calculate artillery data? Calculating how long a shell would travel were complicated in the days before computers. Your own elevation, the elevation of the enemy unit, the humidity, the temperature of the air and of the gun (a warm gun tended to fire further than a cold one) and the wind all affected the range and accuracy of a shell fired out of a howitzer. Accurate data in (detailed maps with detailed altitute notifications, accurate recoinnasance pinpointing the location of the target) helped immensly in this.

• Is the enemy stationary, or the venues of attack he can come through obvious? In that case, the battery may pre-calculate eheir data and even do their correcting fire beforehand. This was the French doctrine, which allowed batteries to be very quick and effective once they had been placed in a spot for a few days, but much less so in mobile warfare.

I'll give a scenario for the longest and shortest scenarios.

Longest

In September 1939, a Polish 100mm wz.14 howitzer battery, part of their division artillery is travelling from one village to the other. A recon patrol have spotted a German infantry battalion making camp in a village 3km away. The recon patrol sends a rider on a horse who reports to the divisional recon battalion staff of their find, who then makes a request by another rider for an artillery barrage to the artillery commander of the division. The artillery commander is under order to move the guns to another point, so he is not certain he can change that order - so he contacts the divisional staff requesting permission to countermand that order. This is also done by rider, as telephone lines between the staffs have not been laid as the division is moving. The divisional commander grants the request, which travels back by rider to the artillery commander. The artillery commander sneds a team of forward observers, equipped with telephones, to the battery of 100mm wz.14 howitzers. When they reach the battery, they relay the orders to the batter commander, who starts setting up his guns, while the forward observers roll out telephone line on their way to the recon battalion. Once they reach the recon battalion, they set up a forward observing post, digging it in and camouflaging it to the best of their ability, and connecting a phone. They phone in to the observer at the battery and make sure the line is connected.

In the meantime, the battery commander has found a good spot to unlimber his guns and arrange for his ammunition supply. He has also put up maps and whatever tools he has to support his calculations and started calculating the elevation of direction (and in the case of some weapons, the powder charge) he needs to use on his howitzers. When the forward observer post reports back, they fire the first gun. Now, it is possible that the calculations were incorrect, or the maps inaccurate so that the shell lands outside the view of the forward observer, and they need to repeat the calculations, but say it lands decently close. The forward observer reports how it missed, the gun is corrected and then fires again. Once the shell has landed on top of the enemy, the gun is left alone and the process is repeated with the second, then the third and finally the fourth gun. As you may guess, the enemy will dive for cover quickly, and they will recognise artillery being zeroed in on them, and look for forward observers to knock out to prevent this. If they manage to knock out the forward observers (say with a sharphooter, an AT gun shell or a mortar shell) the artillery must send out new ones.

Once all four guns have been zeroed in they proceed to pour as many shells as they can as quickly as they can over the target, until the target is destroyed or has moved enough that the fire is ineffective, or the assigned amout of shells for the mission runs out. As you also might have figured out, when the first shell hits the ground, the enemy will start seeking cover, dig in or simply move away if they can.

The whole process can take between 30 and 60 minutes, depending a lot on travel distances between the various units and the enemy.

Quickest

In October 1944, an American 105mm M1A2 howitzer battery, part of their division artillery is travelling from one village to the other. A recon patrol have spotted a German infantry battalion making camp in a village 3km away. The recon patrol reports by walkie-talkie about the enemy to the recon battalion staff, who authorises them to call in artillery support. The recon patrol contacts the artillery battery and requests support. The battery commander immediately unlimbers his guns and lines up ammunition. Both the recon patrol and the artillery battery have very accurate maps made pre-war, with the surveys partially paid for by the US military. The recon patrol reports that the enemy is in square AB 14. The battery commander knows that he is in square AC 12, and pulls up a tape measure which he lays on the map. The tape measure combined with the map gives him pre-calculated artillery data, done by the US military pre-war. Added together with temperature, wind and humidity data the US army measures and provides to its units multiple times every day, the battery commander can commence fire almost immediately. The recon patrol commander with his walkie-talkie (which by this point most commonly is a backpack sized radio set carried by one of his men on his back) has rudimentary training as a forward observer, and can correct the fire quickly, however, with the exact data provided to American artillery, it is often not necessary.

The whole process can take between between 3 and 5 minutes, depending a lot on wether or not the American guns need to be zeroed in or not. If the American guns are already deployed and ready, this could be reduced to as little as 30 seconds.