90 days is somewhat arbitrary, I think it depends on the individual, but presumably 90 days is generally suitable for the majority of pilots. What do you think would the 90 day rule be OK if the pilot was a really old dude?
But there has to be some guideline if not an actual rule. I am out of currency and therefore somewhat wary of myself. It'll be definitely three T & Gs maybe a lot more, and with handy winds to boot.
AIUI these have to be solo; I'm wondering if the safer option of another PPL not an instructor is allowed on this flight by JAR or FAA regs?