Tyres basically start to lose pace after hitting 50% but due to the refueling rule in the game currently, cars tend to usually be lighter at that stage which compensates for the tyre wear and infact makes the lap times faster than someone on fresher tyres that pitted not long ago.
A lot has changed since the latest update in terms of optimum tyre limit but a safe rule of thumb is not to go lower than 40%.
Continue to race and take notes to improve your experience and decision making.