I think it can depend on many factors. If the DJ bike is very different to your usual ride (150mm FS for example) then it may take a little while to adapt, but if you usually ride a short travel hardtail it may take less.
Then another major factor is how quickly you adapt to bikes in general. I’m pretty quick at getting used to new bikes and recently hired a DH bike on a weekend away as the rear wheel on my 150mm FS broke. It was my first go on a DH bike ever, and even though the brakes were the wrong way round and the bike was much longer, lower and slacker than anything i’d ever rode, by the end of the ‘test’ run i was flying and pulled a huge tabletop on the last jump. Conversely, i once took about 2 months of riding every day to get used to a new pair of bars on my bmx as the sweep and rise were so different to what i was used to.
So, in short, it depends 😀