ATP to disassembly of nucleotide ratio
The answer to this is perhaps not as simple as you may think, in part because there are many different helicases in the human body. There is also some passive activity of a helicase that does not require ATP.

It is not clear exactly whether ATP is just involved in the translocation of the helicase or the actual unwinding.

For a RecQ helicase in E. coli, one study found that "The determined tight mechanochemical coupling of 1.1 ± 0.2 ATP consumed per nucleotide traveled indicates an inchworm-type mechanism." For more information see Sarlós et al. (2012). 

Another study (Donmez and Patel, 2008) notes that "As compared with non-ring-shaped helicases such as PcrA (Dillingham et al, 2000) and UvrD (Tomko et al, 2007) that move on an average 1 nt for every ATP hydrolysed, the ring-shaped T7 helicase is three times more fuel efficient." This shows how there can be quite a bit of variation.

Bottom line - you can count on your fingers the number of nucleotides unwound by a helicase per ATP.