The reconditioning of a horse varies drastically. There will be horses that hold their condition better than others. It'll depend a lot on how much time the horse had off and the management of the horse during the time off...did the horse gain weight, did he get turnout, did he get stretched and massaged...
Is the horse one to stand around, or move a lot? What body type is the horse? Just what kind of shape was the horse in before the time off? How was the horse let down? How old is the horse? Does the horse have physical issues like arthritis?
And so on...
The better you know your horse, the better you'll be able to determine how to bring it back into condition.
I think the biggest mistake people make is not remembering that the horse is very poorly designed to carry weight and so it is hard work for the horse to carry a rider, even in walk. A 4hr walking trail ride is so not a cool thing to do to your horse when you haven't been on it in a while.
I actually started a horse back today. I turned him out into an 8 acre pasture, where he spent a few hours roaming. And I mean roaming...he literally did not stop moving in all the time. He was either walking, trotting or galloping at full speed. He stopped a couple times for a drink of water and a nibble of brown grass or hay, and then was on the move again. When he was tired, he came to the gate and I put him away. (He has a double stall and small paddock that he has full access to, and was turned out into a larger paddock several times over the winter.)
He's probably laid out flat on his back right now, dead tired, and he'll be stiff and sore tomorrow. I'll put him out into the big field every day now and by next week I'll probably put him on the longeline for 10mins or so...walk and trot and just encouraging him to stretch and refresh the aids.
I'll immediately start a stretching program with him that will be done once a day and then move to twice a day once I'm back up riding him...which will be dependent mostly on the weather, but I'm hoping to be on him by the first of the month or so.
I'll probably only ride 20-30mins the couple of weeks or so, walking, trotting and some cantering, but my main focus will be forward, stretching, and seeing how much suppleness he's lost. I'll also be taking this opportunity to double check his responsiveness to the aids. I'll expect him to be a bit slow off them.
By the first of May, if the weather prevails, and all goes well, we should be in the full swing of things with him working 4-5 times per week, ring work days 45mins (I don't mess around in flatwork, so 45mins is a decent amount of work and he'll be in a full body sweat), and days we hack out will be 60-75mins.
So, I guess...I'm looking at about 6 weeks to build my horse back up to a regular routine...might take longer, probably won't be quicker for this horse, and the weather will play a significant role. He won't yet be in as good of shape as we ended the season, but we'll be well on our way and he'll be sound.