IME, about 4 weeks before it normally sprouts. You can prune it later, as long as the flowers haven't started to form, but bleeding, and its attendant pests and diseases, might be a problem.

I did mine at christmas as it if fully dormant. once it has 'broken' bud it will 'bleed' sap if you prune it. this will weaken the plant. so if you need to do it asap and hope it stays dormant a little while longer.

This is why when ever you see old footage of French peasants pruning back their grapes by hand, it's in the biting cold winter as you can always see their breath!

the victorian's used to have the 'boy' prune them on xmas day. This was a common occurance and is well documented I suspect it was more to 'keep them' in their proper place rather than preventing pruning.
its also an excuse to get out of the kitchen for half an hour on xmas morning for me. :o)