Any time in the next week or so.

If you live in the west or south, south east, it should be in more or less full leaf by now.

Have a look at the buds - if they look healthy and turgid to the touch then the tree is fine. If you have only planted it recently then it may be putting down roots before opening its buds. Also how much light it gets also afffects when it leafs - more sunlight hours the quicker it leafs.