When I first took up Aikido I simply overdid it. I practiced every week-day and two or three days a week both morning and evening practice (I was unemployed at the time). It was fun but my motivation was my desire to grade quickly and within a little less than half a year I lost my 'drive'. It was too much. I coulnd't process all the information I gathered and I found my self more and more confused. In the end I had to stop but now I'm back after a 3 year break and I have found a level of practice much better for me. Twice a week (1½ hours each) plus two or three weaponsclasses (1 hour each)
I don't know if my experience translates to everbody but for me there is definately such a thing as overdoing it.
Whatever you do - have fun!