Sparring in the main ranges massively depending on students, whether there are any fights coming up etc. Generally speaking I've always used the following guidelines: -
1. General sparring for the whole class should be light with students focusing on developing their timing, footwork, distance, defence and guard - I always tell them no one should be worried about getting hurt which prevents students from experimenting with techniques
2. Students wanting to have first full fight in ring need to be tested to see how they can cope when faced with adversity!! These sessions can be quite brutal and either change the students mind about fighting or strengthen their resolve. The best thing to come from these sessions is when someone thinks they are ready too early and then realise they need quite a bit more practice!!
3. Experienced fighters need to push themselves, dependent on the level they compete at. As a general rule of thumb if a fighter has a 3 round fight they should be able to compete at a good pace (not too hard though) against 3 sparring partners for 9 rounds, 5 round fighters should have 5 sparring partners for 15 rounds etc etc. Also paramount that they travel to other gyms to spar with other fighters of their level or higher to gain experience of sparring with people they don't know to get used to the adrenaline dump.
Hope this helps!!