You can do it several ways to be honest.
The syn limit can depend on a person's weight - and men (if I've remembered right) get up to 20 syns a day.
Generally though, yes - everyone gets up to 15 syns each day.
If you think of it over a weekly basis, which is how I usually do it, you've got 105 syns to use over 7 days. You can use them how you see fit.
What I don't advise, is that you save all 105 syns for the weekend. Use at least 5 each day, and then you've got the rest to play with. Using 5 each day helps stop you feeling deprived mid-week if you wanted to use all of them at the weekend.
The other option, and one that I've used plenty of times, is to Flexi-syn.
Flexi-synning is where you decide that you are going to use, for example, 50 syns on a Saturday (say you have a night/meal out/takeaway planned) You then have up to 50 syns worth of food/drink and when you hit that 50 syns limit, you switch to syn-free things.
For the rest of that week you follow the plan as normal, and use your 5-15 syns each day.
You can't be guaranteed a loss, but you at least know you've stuck to plan every other day and it isn't as hard to give up those naughtier foods
I've done both in the years I've been on SW, and depending on what my plans are, both options have worked great. I've not been left feeling deprived of anything, but I've also not felt guilty of over-indulging on anything.
I've done what Cinnamon says lots of times, but once I actually added up the syns I'd eaten/drank over one day and it was beyond shocking. I think I used almost 2 week's worth of syns in one day??? Eating freely can work fine for some people, but it just doesn't for me - it ends up sending me off plan completely and I get into that vicious circle of feeling guilty and eating my feelings