I was wondering if i bough out the shop or at least the ammo will it ever regenerate so i can buy more? or will i have to go to a different store? If it does come back how long? a couple days?

72? 73? Hours and they will restock.

I have not verified this, but someone poked around in the GECK and the Restock cycle is 7 Days, and it seems to happen on Sunday (game time).

Note that stock is slightly random. Don't expect them to always have the same items.

It definitely seems quicker than that to me. Maybe different vendors have different timers. Or different items have different timers /shrug

Yeah I have noticed refreshed lists in as little as 3 days.

This thread I saw this in was talking about Gun Mods, so those might be on a different schedule. It was also specifically talking about Goodsprings and Novac. 72 hours has always been the schedule for everything else since the days of Oblivion/Morrowind/Daggerfall/Fallout 3.

The Gun Runners right by The Strip is constantly resupplied. It's about every 48 hours. That's weapons, mods, and ammo. All of it's replenished. The mods change just about every time.

How long does is take for npcs to respawn?

same as FO3: 3 days?

it seems its longer for some and shorter for others