It depends on your host, the plan you're on and how long your time commitment with them is. Basically, the best time to do this is just before your next renewal period. You should start investigating the process about 60 days before you renew. If you're planning on doing this, you'll want to log into your current hosting account and disable any kind of auto-renewal function that's active. This will stop the current host from billing you if life keeps you busy and the date slips past you.