A) There's essentially an unlimited number of primes. Even when youre talking numbers hundreds of digits long, yoiu'll still find primes in them pretty regularly (every couple hundred digits or so).
B) Primecoin isn't even trying to find the biggest primes out there - indeed if it was and did, Sonny would qualify for some pretty big reward money
means (2^332.xxx.xxx)-1 , to confirm that a number this large is prime with LL would need 2000 hours with a GTX Titan or 3400 hours with an 8-core CPU
now you have to see that the chance to find a Mersenne Prime in that range is 1 : 370000 (many candidates are proven composite with trialfactoring)
so normally it would need ~168.000 years for one person to find a 100,000,000+ digit prime, try your luck