In reply to pine85uk: the answer, needless to say is "it depends".
The North side of the valley (i.e. South facing) routes can come to an end quite early e.g. toward the end of Feb, one always needs to be careful about their conditions as they catch the sun, especially on the upper slopes.
The South side of the Valley (i.e. North facing) can last quite a bit longer, they are also often well protected by tree-clad slopes from above (but not in all cases, take care). Their suitability will vary with air temperature, both at the time of climbing and for some days before. This can vary quite dramatically, almost as much as in Scotland, but climbing on the North-facing side can last well into March.
This is all apart from the higher mountain routes near La Grave, which can continue into April and beyond, but might well need skis/snowshoes to approach.