I am just a small home baker, I do a few weddings a year, maybe 10. I personally ask for a deposit of half and it depends on what I need to do for the prep work/ordering supplies to how far in advance I ask for it. There is a wedding in Oct that I have to order dummy cakes for so I asked for the deposit in Aug. but if it is a cake that I source everything locally and it is a simple order I ask for a week or two in advance just so I can cover the cost of my supplies.
Depending on the time frame in which it is cancelled I would most likely give a refund, I have yet to have this happen. But if it was the day before or something I would be hesitant to refund the deposit unless it was something tragic like a death or something. But like I said I am no expert, just a beginner home baker =) Good luck!