I haven't turned my PS3 off, except to move it, in months. Turning it off and on hurts it more than consistant warmth. Just make sure it has good air circulation around it and blow some dust off into the vents every week or so.
I probably did more damage to my PS3 in the first week I had it by insisting my kids turn it off every time they were done with it than in the last year.
There have been posts asking this same question, or similar enough, in the past. Use search.