This question is very hard to answer, because we have a bit of a problem of sources. We can't assume that the surviving ancient texts - with their focus on particular states and particular events - actually tell us about all the wars and battles that went on. In fact, some major engagements are known from little more than off-handed comments, and we can only guess how many more there are that we simply don't hear about at all. While we can go some way toward a reconstruction of how often a Spartan or an Athenian citizen might have gone to war, we cannot do the same for a citizen of a more average city-state like Sikyon or Orchomenos. How can we tell what was typical?

As a matter of fact, even Athens, by far the best attested city-state, gives us trouble because of the patchiness of the evidence. An often-cited factoid is Yvon Garlan's calculation that Athens was at war for 2 out of every 3 years of the Classical period. This is constantly repeated but I don't think it has ever been verified (Garlan does not cite his source for the figure). Which wars does it include, and how certain are we of their start and end dates? Could there be wars fought by Athens that have left no trace in the sources? (Answer: almost certainly yes.) Were all these wars "hot" wars, or do they include years of passive stalemate and negotiation? The closest thing we have to a complete picture covers only a limited span of time (431-404 BC), and that is precisely because Athens was embroiled in an epoch-making war. How representative are the data for this period?

All that said, it is almost certainly not true that (as many modern scholars claim) the average Greek citizen marched out to battle every summer. For one thing, no state on record comes even close to that frequency of belligerence. For another, the vast majority of known campaigns did not involve the full levy of all citizens of military age of any city-state. A state of war did not automatically imply a call to arms. Most of the time, there would be a draft from the Lists or some other form of citizen registry, which usually only included the leisure classes. The majority of the population would not be not called up unless there was a pandemei or panstratiai levy (i.e. if "all the people" or "the whole army" was called up). Only a leisure-class citizen might find himself sent out to fight in every single war; as a regular farmer, you might guard the walls of your city, but you would only march out if your state was making a point, or if it was in dire straits.

On top of that, many campaigns did not actually end in a pitched battle. Some ended at the border, if the omens were bad; others involved unopposed ravaging; others saw skirmishes between looters and cavalry, but no major engagement between the armies involved. As an ordinary farmer, you'd soon be getting keen to head back to your farm, and so these campaigns couldn't last too long. If there was more warring to be done (for example if your army was besieging a city), this would be the point at which common folk like yourself were sent home, and the men levied from the Lists would make camp for the long haul. In none of these cases would you actually go into battle. Even if we assume that there were many more battles than we hear about, major pitched battles were quite rare.

Take the date you've picked, for example. I know of no battles that took place anywhere in the Greek world in 500 BC. In fact, there are no battles that can be securely dated to any year between 507 and 497 BC. No doubt there were various conflicts going on, but no source gives us any specifics. You presumably wouldn't have been able to grow your crops in peace for all of those years, but at the same time it's safe to say you probably wouldn't have been called up more than once or twice in that period.

In times for which we have better information (i.e. the late 5th century BC), it would be fair to say that you might get called up every year in wartime, but mostly for the less dangerous jobs. For example, during the early years of the Peloponnesian War, the Athenians invaded their hostile neighbour Megara twice a year with the full levy, just to burn and pillage anything they could find. None of these invasions were ever opposed, because Megara had no hope of mustering anywhere near enough men to confront the tens of thousands that Athens threw at them. So, if you were Athenian, all you'd have to do was walk around stomping on grain and setting fire to farmhouses until it was time to go home. If you were Megaran, you'd get your revenge if you were part of the two-thirds levy the Spartans typically required your city to send for its invasion of Athenian territory each summer. It was practically certain the Athenians would not come out and fight.

How much of the time was wartime is an open question. But even during the Peloponnesian War, there were several uneventful years, in which Sparta did not call out the levy of the Peloponnesian League and Athens had only men from the Lists in various expeditionary forces. In such times you, as a regular farmer, might escape military duty altogether. And if your city-state wasn't a member of the League or a subject of the Athenian Empire, you might be fighting your own petty wars, but otherwise you'd be left mostly alone.

It is anyone's guess how often you would actually be involved in a battle. Given all the factors I've mentioned, regardless of your city-state's foreign policy, I am of the opinion that the answer is probably closer to "once or twice in a lifetime" than "every year". However, given the lack of comprehensive source coverage, and the fact that historical accounts sometimes casually mention several battles in the course of a single campaign, my guess may well be wildly off the mark.