There is no 'have to' as far as time you put in. You can put in a day a month, if that's all you have & you'll get there... eventually. The more time you have to put in, the quicker you can get things established. The more regularly you can do it the easier it is to get things solid & reliable.
As a hoof care practitioner, the more I study, the more convinced I am of the utter importance of lots of regular exercise for horses, starting as early as possible tho. 24/7 turn out is a great start, but in conventional horse properties, horses have ample food & water available in close proximity, so tend to be rather sedentary. Ensuring they at least have company to play with & taking them out to exercise as much as possible is great for helping them develop strong, sound bodies & hooves.
For horses of any age, especially those learning lots of new stuff, short, positive sessions are best - anything from 30 seconds to perhaps 10 minutes, depending on what you're wanting to teach & how the horse responds. This doesn't mean that's it for the day or the hour even tho. There's no reason you can't have 1 minute 'on', with 1 minute breaks in between. So in 2-3 days, you could potentially have hundreds of sessions.
Assuming the foal knew nothing, I would concentrate first & foremost with getting him accustomed to me & my 'toys'. Get him used to feeling ropes around him, in a non-confrontational way. Get him used to wearing a halter & yielding to pressure on the lead, pressure from me. Desensitise him to other toys, tools, etc.
After you've taught the foal to yield well to pressure & he's reliable, then you can teach him to tie if you wish. **If you're going to teach a foal to tie up, do not just tie him by the head, but use a rope around his butt or belly as well, so if he fights & pulls back, the pressure isn't just concentrated on his delicate neck. I don't think it's safe to tie a horse solid until he's reliably 'tying' on a yielding rope - see Parelli's method, or equipment like 'The Clip' or 'Blocker Tie Ring' for safe eg.
Once you've taught the horse to respond to your control & be comfortable with it's surrounds, teaching it to pick up it's hooves, allow touching everywhere, allow you to play with it's mouth, nose & ears, syringe apple sauce or some such in it's mouth.... all those things that are necessary for stress free management, hoofcare, vet care should be done ASAP. As foals can develop hoof pathologies in conventional domestic settings by only a few months old, it's imperative to be able to provide good regualr hoofcare ASAP. If you have a float/trailer handy, I'd be teaching the horse early to be comfortable in there too, in case you want to(or have to in emergency) take him somewhere.
Basically everything else that the foal will need to know in life can be started early. The more you do when they're young, the easier & more reliable & confident they'll be when it comes time to 'start' them. Because effectively, you have already started them, aside from weight bearing & the likes. I like to take young horses anywhere & everywhere, as well as teaching them at home, so they get accustomed with different environments, different experiences, sights, sounds, etc.
As I work with young horses in the above manner, there's no real big event of 'breaking' the horse, nothing big for it to have to have intensive training for at age x. While I will often lie accross the back of a 2yo, and often take short walks, sometimes trots on him, depending on the horse & his development, I do not agree with aiming for prizes in futurities or the likes, but rather wait until the horse is 3, 4 or 5yo(depending on type/physical maturity) to do much at all under saddle, because I want the horse to be pain free & not put unhealthy stress on his still growing body.
Anyway, that's my ...$2 worth! Hope it's of some help!