I just started my 7th sketch pad (or, more grandly, visual art diary). Adding in pages I've drawn outside these, I've filled something like 440 pages in the past 3 years. That only averages out at around half an hour of drawing a day. Most of what I've read about getting good at anything indicates I'll need to up that to something more like 2 or 3 hours, if I'm serious about developing.
That said, I've drawn more in these past few years than the previous 32 years put together, so I'm allowing myself a little pride, for improved effort if nothing else :) And nothing breeds quantity better than statistics, so I'm sure my latest pad will be full in no time!