What was life like in Japanese prison camps in WW2?

Depends where you got taken prisoner, when you got taken prisoner, and by who you got taken prisoner. The only thing consistent about Japanese treatment of prisoners was its inconsistency.

For example, if you were a US Marine Corps embassy guard in China, you had developed a relationship with the Japanese embassy guards. So when they came over to take you prisoner, they were generally apologetic about it, and you were treated pretty well for a prisoner. For these men, rations were equal to those received by your regular troops (basically, rice and some sort of protein) and while you were expected to do some labor you passed the time relatively easily.

If you were fortunate enough to be sent to Japan, while you were expected to do a lot more labor (steel working, construction, etc.) you were also treated somewhat decently, and you also were offered the same sort of healthcare that the civilians had at the time.

If you weren't lucky, you ended up in the tender care of whatever improvised or semi-improvised solution the army had to take care of prisoners. The infamous Bataan Death March, for example. The Japanese commander had planned on taking some 20,000 prisoners, and had arranged for logistics to feed this group along a path to a camp constructed for them. However the problem was the amount of prisoners was more like 100,000. The Japanese logistical situation was also a total mess. They lost control of their soldiers, who often would execute stragglers and beat the prisoners. Physical violence was endemic in the Japanese military, and the lowest ranking troops would beat prisoners to vent off their steam.

Or maybe you were one of the ones that got captured in Southeast Asia, where you got to work yourself to death building a railway to Burma.

Or maybe you got thrown onto a hellship, where you were treated as cargo among many prisoners, some of whom were diseased. Maybe you'd die from the contagious disease, or from malnutrition, or of course from Allied bombers mistaking your prison ship for a cargo ship and bombing it (which happened tragically several times).

Ultimately prisoners were fed and treated according to the whims of their captors. The Japanese military was very hierarchical, but at the same time the high command had very little control over the situation on the ground. So most decisions were made by junior officers and NCOs, who were inspired by old samurai ways (see gekokujo) and disdained prisoners. There would be rare moments of kindness, like the story of how a Japanese soldier heard an American prisoner singing and gave him food and anti-malarial medication (secretly, of course) but on the whole it was very brutal.

Generally speaking, most troops were captured by a swift Japanese advance that cut them off on an island with no supplies or means of escape. Like in Singapore, the Philippines, and the Dutch East Indies.