Typically how many days would a cheap ovulation test show positive?

[kitkat2016]

Hi there,

I just wondered if it was possible to find out if typically there was a set number of days or maybe just one day that an cheap ovulation test (from the internet!) would show positive?
Sorry if this is a silly question but Im confused about exactly how the ovulation tests work and seem to get conflicting advice when looking it up on the internet so just wondered if there was an average amount of days a test would show positive.

Also I wondered does it typically change days or stay constant? I have a 30 day cycle and from the signs it would appear that I typically ovulate on day 14 or 15 but does this change occasionally?

Any advice greatly appreciated