I never really stayed in one map for too long but now that you've mentioned it I do recall thinking things were taking too long to respawn during some specific mon grinding.
Z-A has definitely fixed this with more intuitive respawn mechanics, but it's got much more of a focus on battling and less on collecting this time around, so it isn't for everyone. Much more fun than the main series has been in a while though!
I obviously can't comment on everyone's experience, but for the few people I know that did get cancer (including my mother) they were all scheduled scans within two weeks and had results delivered within another two (or less especially for the first diagnosis).
In my mother's case everything moved extremely quickly, due to the doctor suspecting something serious the first scan was scheduled in a few days and the results were back before a week had passed.