As stated in the tooltip, the timer for the fitness peak is acivated 8 weeks from your objective. With that in mind, you can calculate when you would get 100% by knowing these thing:
3.0 per race
2.3 per days without racing if your fitness is above 95%
1.8 if it's lower than that
1.4 if it's much lower i.e., 89% fitness
I think it even gives 1.0 if the fitness is below 84%
The meta in order to keep as high fitness as possible is to train really hard at the start then once you achieve a fitness of around 97%, you can then train easy all the way until the end of the season wthout losing your fitness level in order to get the passive bonuses during races due to high fitness level. The caveat though is if your fitness level is very high, you would fill up your fitness peak bar much faster. (6 weeks without racing if your fitness is 95%+ is already 96.6/100 of the fitness bar). So how do you circumvent the issue? First thing is to pick an objective much later than the race you target, let's say tour of Pologne or San Sebastian then once the fitness bar is activated, you can then cancel the Tour of Pologne then replace it with Tour de France).
After you calculate it, you'll realize that you will enjoy your fitness peak only at the latter part of Tdf since the Tour of Pologne race is still 29 days after the start of Tdf. This first method works pretty well with all kinds of races except Grand Tours. So what do you do now?
Here comes the second method. Most likely Tdf isn't going to be your first objective of the season hence it won't be your first activation of fitness peak. So the second method involves timing the end of your first fitness peak such that it synergizes with the second. Yes, the fitness peak timer is activated 8 weeks from you objective but only if the bar is empty, If you're still on your first fitness peak then the bar wouldn;t fill up. To give you a vague example, let's say end your first fitness fitness peak 6 weeks before Tdf then don't race anymore then voila, the timing is almost perfect.
Lastly, is something I wouldn't recommend but it can still work. I made a mistake once by not timing my first fitness peak to my Tdf fitness peak. So I emptied the bar and realized that I will peak way too soon for the Tdf. What you can do in this kind of situation is use the rest feature. I don't recommend it since it messes up the calculation and makes you weak for far too many races in my liking. I would like it more if it doesn't have the resumption of training week after the rest week. So basically it lowers your fitness level below 90 in exchange of resetting your fatigue level. And the reason why I don't like this method is because you'll train hard again in order to reach very high fitness for the race. Hard training has pretty much the same fatigue level as racing days hence doing this method while preparing for a grand tour sucks so since it makes you very fatigue at the 3rd week unless your recovery is high. But at least it's still better than peaking way too late.
If you want to peak in January races lke Tour Down Under then I'm afraid it isn't possible due to the rest week nonsense the game force you to undergo after January 1. Peaking in early Feb races generally means you have to peak a much earlier race let's say Tour down Under in order to help fill the bar up quickly, otherwise you'll miss the timing due to your low fitness level. Always select very high during preseason, easy training is essentially a rest day.
Timing fitness peak is one of the most enjoyable things for me that's why I almost always max it first since it gives you so much control. I hope this helps.
