In reply to Ian:
Can't comment on broken sternum - fortunately never done that one.
In my youth, broke a toe (same one!) a couple of times playing rugby - was out for nearly two months so I would guess four to six weeks for a non-contact sport, e.g. straightforward climbing. I also seem to remember the toe ached quite a lot during and after exertion for months afterwards, particularly in cold weather.
Torn knee ligaments. You may not want to hear this but I tore them badly playing rugby and it was six months before I could do any reasonably energetic walking, e.g. Pyg Track up Snowdon, and nine months before I could play rugby again, which I would guess is the nearest comparison to serious climbing.
Main lessons learnt from the above:
1. Get good advice/recovery exercises from a physio right from the start of your rehab, even if you have to go privately - and work really hard at the exercises suggested. It does pay off.
2. With knee ligaments. Even when they start to feel strong again when moving normally, any sudden sharp movement, e.g. a slip, can tear them again. So err on the side of caution when getting back into action. A couple of extra weeks weight training to further strengthen the knee could prevent a recurrence of the injury. Not very macho advice, I know, but it could pay off in the longer run!