In reply to jimbob1992:
Yes, the season usually runs through to April. In fact the likeliest month for the season to end is May, though it can also end in April or in June--or if you don't mind having only simple ice-gullies to climb, September
There's a good essay on finding the best conditions in the intro to the SMC Scottish Winter Climbs volume, ed. Simon Richardson. Or look at his recent Chasing The Ephemeral.
In brief--for some things April is likely to be the best time of all, in particular for north-facing ice routes high on the Ben: think Indicator Wall, Point Five, Hadrian's, Orion Face, Smith's, Comb Gully, Green Gully. Also a good time of year for "Alpine" conditions, i.e. hard gully-ice, stripped-back snow, and sun-warmed rock: think Aonach Beag NE Ridge, NE Buttress, Observatory Ridge. (This pic was not taken in April, but it does show what Obs. Ridge often looks like in April:
http://www.summitpost.org/observatory-ridge-tim/148787/c-150801 )
(Sorry, rereading your post I see you're talking about the Cairngorms. But you've a better punt at good conditions on the Ben at that time of year, so no apologies for going on about the Ben
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Post edited at 22:10