When is a good time to ask to go part time?

[Monkeybar]

Just about to go back to work in a sales job. Some of my colleagues with kids do a 4 day week, I'd like a 3 day week (I think, but won't really know til back). Is it better to ask for reduced hours up front before starting back? Or might that imply that I'm not as commited to the job as I was before I went on Mat leave? But once I'm back full time, does it make it harder to ask for reduced hours?
On the plus side, sales territory has been reduced while I've been off. On the minus side, the company has launched some new products and is after high profile! Any suggestions, please?

You could ask to start back on a 3 day week with a view to increasing to 4 later. You could call it a phased return to work.

Thanks BadHair...I've already had that approved, but don't know how I'll feel when I have to go 5 days after 2 months.