Although I have been only been training for a bit over a year now, I've found that - at least for me - keeping up interest and drive is a balancing act: If I go too often I simply get beat. If I go too little the stress of college work and research consumes the time that used to be spent at the dojo.
I've finally found a nice trend that seems to be working rather well: open-hand training two days a week and bokken once a week. With my busy schedule this works out well for me.
I also completely agree with those who say that sometimes you just need to take a break and explore other areas. Whatever you learn can always augment your training in one way or another and everyone needs a change of pace once in a while.