He probably did most of his painting at PIFA when he was enrolled there (including the second half of S4). I imagine he might have done some painting when he 'lived' with Daphne in between his stints at PIFA. And then he went off to Cali....when he got back, I'd say he didn't paint...just sketched and had his fancy computer to work on. That's when he and Michael worked on the marriage issue of Rage, so much of his time would have been devoted to that. After that, I think a lot of his unhappiness and decision to leave Brian was caused by his lack of creative outlets and uncertainty about his future. Moving into his studio gave him a creative (fulfilling) burst energy that he'd been looking for/craving.
does any of that make sense? in my head it does...