[HelloBaBeS]

Hello
I am setting up an awp server and liked that the map does not change at the end of the 15 rounds of casual but rather at the end of 30 minutes.

[Cruze]

Didn't understand which way you want but:
1) If you want that map should change after 15 round then
mp_maxrounds 30
mp_timelimit 0
2) If you want that map should change after 30 mins
mp_maxrounds 0
mp_timelimit 30

This values should be changed in gamemode_*.cfg where "*" stands for the gamemode you're running (casual, competitive, etc)
AWP gamemode is generally runned in casual gamemode so you should change cvar value in gamemode_casual.cfg (csgo/cfg/gamemode_casual.cfg)