Every performer gets two songs* (or about 5-8 minutes for comedians).
Once we've gone around all the performers once (i.e. the First Half) then we'll go round everyone again for a Second Half (in the same order they played in the First Half).
The number of songs each performer will have in the Second Half depends on how many performers there are to play (e.g. the more performers we need to get through, the less songs each performer will be able to play in the Second Half).
* N.B. If it is a particularly quiet night, each performer may get more than two songs in the First and Second Half (e.g. three or four songs each half).