The original Skewes number was merely an upper bound before which the condition Li(x)<pi(x) must occur at least once; this number (Sk1) was e(e^(e^79)), or roughly 10(10^(10^34)), in 1912. By 1987, this number was reduced to approximately 8.185*10370. Both of these numbers depend on the Riemann hypothesis being true. Sk2 is the upper bound if the Riemann hypothesis is false; it is estimated to be 10(10^(10^(10^3))).


Source: http://mathworld.wolfram.com/SkewesNumber.html