Title.

I was wondering because I know that normal NPCs don't respawn when they die, but what about town guards?

Of course. If you killed every guard in Skyrim, than you would be able to do anything you wanted with no penalty.

It takes a while for a majority of them to respawn. After racking up a 60,000 bounty in Riften, I only ever saw any of the town gaurds if I went into the castle. Otherwise I came and went without any interference.

They do...not sure on the time limit though.

Npcs, Creatures, Items, Normal Areas respawn after 10 Days (240 Hours). But be aware that each time you re-enter a cell you reset the timer. If you go back to that town every nine days the town will never respawn guards.

Think of the patrols you see out in Skyrim. What are the odds that one of those soldiers is going to take an arrow to the knee? It takes time to get enough guards fit for duty.