How long does it usually take for a pelicacle to form?

how long is a piece of string? ive had pellicles form in less than a month and currently have a kriek thats over 15 mths old that has never formed one at all, it all depends.

It really depends on the amount of oxygen that was introduced to the fermenter. Is this beer in primary or secondary?

Its in sexondary. I pitched cal ale yeast first then let it ferment until it hit 1.020 and then recked to secondary and added the sour mix blend

What is this secondary? I must get one!

It will take some time. I have had sours get pellicles after 3-4 mos, and some never even really get one at all. I have also had lambics in primary form pellicles within 1.5 months too. Each sour beer with the different microbes have a mind and plan of their own which we cannot really control. So, just be patient, and don't even have a taste of your beer until it is at least 6 mos old.