Ten million digits
After the discovery of M38 (the first megaprime or prime number greater than 1 million decimal digits) in June of 1999, the next EFF prizes for prime numbers was ten million decimal digits.
Prime95 had an optional worktype to test numbers that were at least 10,000,000 digits added (exponents of 33,219,281 and higher). When first made available and handed out by PrimeNet, the Lucas-Lehmer tests of these exponents took over a year on the fastest processors then available for personal computers.
In a period just over 2 weeks in summer 2008, the first two Mersenne primes greater than 10,000,000 digits long were found, M47 and then M45. In April 2009 a third prime greater that 10,000,000 digits was found, M46. By the end of 2010, all exponents that would produce a number less than 10,000,000 digits had been tested at least once.