Took me about 8 years to get close to even. A good exercise for the left hand is the shuffle pattern, you know, dotted 8th notes. Do them with your left hand on a left hand ride (so you aren't crossing your arms) as fast as you comfortably can, as long as you can, till fatigue. You might get some disagreement there. IMO, that's the only thing that gets it stronger, fatigue. I've been regularly fatiguing my hands for almost 9 years with great results, so naturally I recommend it. Tension and fatigue are 2 different things. Tension, generally speaking, is not desirable. Fatigue on the other hand is how I get my muscles to do more than they currently can. I play my weak hand until fatigue....and a few days later I can go a slight bit longer until fatigue, and so on and so on...
Basically what I did is totally focus on my left hand. It still takes quite a bit of time. Virtually all drummers have this issue. With drums, ideally, you need your weak hand to be just as strong as your dominant hand. Tall order. Translates into many many many hours of left hand focus.