How many months my child realistically sleep in the carrycot before it’s too small?
As the internal dimensions of the Prime carrycot are circa 80 x 35 cm, this is actually a very roomy carrycot. Just for a reference, my 4110 g (when born) son was in a 73.5 cm long bassinet up to his 7.5 months (we was 7.5mo in the middle of winter, with a footmuff). That means that, if your baby is not extremely tall or large, or you are not overly demanding with the carrycot space (like needing 10 cm all around when the baby is inside), this carrycot should last you at least 7 months, but realistically 8-9 months plus (again, depending on the size of your baby as well as your requirements and the season).
Do remember that babies like to be a little tight in their space - that is how they were in your womb, so it makes them calm. A smaller carrycot is not a problem for them, it is more for their parents.
And a P.S.: Here is our carrycot guide with all you need to know about prams' bassinets. It will tell you all about carrycot's features, size requirements, and other things to know what to look for (and what's not that necessary after all).