Assuming we're talking about males...
1) initially 10-14 days. However, they are susceptible to abscesses for many months afterwards. One of mine had abscesses relatively soon after his castration, and he was still getting them after 6 months.
2) Death. Be it while under anaesthetic or post op due to complications. An experienced vet with a good nursing team, and close monitoring after the surgery by you will help reduce the chances of anything bad happening. Being aware of the potential problems and being prepared will help a lot (having some syringes and critical care on hand incase of anorexia etc.)
3 & 4) research your vet well. Use a recommended vet if possible. Restricting rest after surgery can aid recovery. Mine recovered in an 80cm cage so minimal running around could be done. A selection of foods (veg, nuggets, "treat" hays, syringe food) incase you need to encourage them to eat. Probiotics are also a good thing to have to hand.
You would expect them to be sent home with pain relief and most likely antibiotics. They're usually given antibiotics prophylactically as the wound is so close to the ground and they do bum drag and things.
Fibre free bedding is important - fleece or vet bed is good, ideally in a light colour so that you can see if there's any bleeding. A couple of pieces will be needed so that the cage can be kept very clean to minimise any infection