Both @ScarDuck14 ans @Nuvolarix are perfectly right.
I just wanna share my experience. I tried both long and full sessions in several releases of the game - and I preffer the full session 'cause, as Nuv said, it allows you broader variety of pit-stop strategies (not to mention a greater chance of changing weather midsession) and I think this adds a lot to the experience. As you @Ansakura, I tend to play F1 only during the weekends and I preffer to save midsession and finish it latter so I can enjoy this variety of strats/weather rather than doing a 50% race and being limited to less strats and weather changing.
Again, just sharing my point of view. Hope it helps somehow.
Cheers.
Dan