Its different for every woman!

The cycle length shouldn't really change how early you can test as I think in most cases it is always roughly 14 days between ovulation and AF (or no AF!).

However - your levels of the hormone that the HPT looks for might not be high enough until after AF was due, which means that if you tested before AF is due, it would show a negative.

There were some statistics in the pack of HPTs that I bought (Crystal Clear) that said something like (don't quote me) only 50% of women get a positive 5 days before AF is due, increasing slowly up to 93% or something on the day before AF is due.

If you want to be 100% sure, wait until on or after AF is due!

That is if you can be 100% sure when AF is due!!!

Sorry to not be more helpful!