Most days I have to much to do and just do a bit of casual solving. If I do have time I put in a solid hour and a half or two hours, but that happens only once every week or two. I'll put in an hour or two every day for about a week before a comp, but usually I don't have time for that. I've voted less than 15 minutes but only because my practice is so spaced out, not because I practice in short sessions.