lime around here usually lasts about 3 years with sandy loam soil. I still take soil samples, but its like clockwork 1 ton/acre every 3 years. I use dolomitic limestone for the added magnesium content. Powder vs. pelletized shouldn't make any difference IF you apply the same rate of each one. Most of the pelletized lime I've seen comes in 40 or 50# bags, and it's hard to get enough lime down like that.