Gamespeed and style (Classic, Hardcore, Turbo) make a difference. Classic 100% gamespeed is real-time. Hardcore/Turbo 100% gamespeed are actually 10% faster than real-time.
Item respawn times are quite "even" numbers in game seconds: health packs 20 seconds, health vials, armor, pads, boots, regular weapons and ammo 30 seconds (volatile ammo 15 seconds), redeemer and shieldbelt 60 seconds, big keg 100 seconds, amp 120 seconds.
Divide the respawn time by the gamespeed and again by 1.1 on Hardcore/Turbo mode.