Visa may run out whilst in China as it is our last stop

My friend and I are travelling around Asia for exactly 3 months and 4 days starting on the 27th of Feb. We will be applying for an express visa 9 days before we depart for Asia (China is our last stop) as my friend is working in Russia till then. I'm curious about the time the entry permit starts; does the entry permit validity begin once we enter China (approx. 15th of May) or once we have collected the entry permit from the embassy in London? If it's the latter does that mean we have to apply for a second entry permit whilst there as we will be overstaying the 3 months validity?

Thanks,
Regan