just kidding
if you get to sail every weekend you will probably be there within a couple of months maybe earlier. Some folk take longer, some pick the basics up really quickly, some need a bit of a kick up the bum and they progress quicker than left on their own. Often the waterstarting comes at a similar stage to learning to get into straps, harness and getting on the plane. certainly gives you more sailing time once you get the hang of it because you get less tired & sore than with uphauling, especially with bigger sails
if you start to get into freestyle you'll re-visit the joys of uphauling and develop a new appreciation for it and with tacking too
It depends on whether you are teaching yourself or if you have a learned friend or instructor to help. Months for the latter, possibly years if you are teaching yourself.
Start with beach starts, particularly concentrate on using the rig to turn the board slightly off the wind before you step on. Don't fight the sail, allow it to flip until you have it where you want it. You can probably get this in one session if you have someone to show and guide you. Then do deeper and deeper beach starts until you are up to your neck in the water. Then the only difference between that and a waterstart is that in a full waterstart you have to swim the rig into position and you don't have the bottom to push up against to raise the mast.
Good luck, and keep trying, it's worth it!
Windxtasy has the right idea. Just work up to it from beach starts. Use a sail that doesn't have a huge luff sleeve like a cambered race sail. Wearing a life jacket or impact vest will help conserve energy as you have to swim the sail around.
When I learned how to water start I found lowering the boom so it could sit on the back of the board was a great help. With the shorter boards and more rearward mast tracks of today its not as easy to do this. Make sure you keep your uphaul rope on your rig. Even when you learn to water start there will be times you fall in and the rig is in the complete wrong position for a water start and a quick uphaul where you can get the rig in the right position as you fall in, will save time and effort swimming board and rig around.
I've sort of gotten out of waterstarting practice but now with a dodgy lower back, I'll try to minimise uphauling in the future.
It's not quite the same as a beach start, cos u aren't standing on anything. Unless you're very well powered, the trick is to get the sail up high and forward without too much of your weight on the boom; to achieve this, your body needs to kind of lunge forward, close to the board, so it won't take so much force to pull you up. At the same time try to pull the board under you with the back foot...oh hell you just asked how long it takes to learn, not how to do it...sorry...
...anyway, as a general rule, I find that if there's enough power to plane easily, then I can waterstart easily. If I'm overpowered, waterstarts will be super fast and easy, and if it's marginal then waterstarts will be slow and painful...and wear a bouyancy aid so you can use your energy for other things besides kicking.
I'm teaching my gf to sail at the moment, and she's also opted for Decrepits "Short and Hard" way of learning.
2 sessions on a Starboard Go, a couple of sessions on a TurfDog, and then she announced that she didn't want to uphaul anymore.
A couple of sessions beach-starting a 99 litre (she's only small) and then onto waterstarting -she got her first one in her first waterstarting session.
She does of course have the (dis-)advantage of me teaching her, and skinny masts/freestyle sails, but I reckon we'll have her waterstarting consistently by the end of the month (depending on how much sailing we get done).
It's never too early to learn, but I would advise trying it on a landsailor first (sitting on a low crate helps when first trying it) and with someone who knows what they're doing instructing you - much less exhuasting...
(PS - when I started I had rubbish equipment and spent months teaching myself how to do it - it was exhausting!)
Good luck!