I asked the same question not too long ago, but unfortunately received no answer.
IIRC, the default settings for scrubs in TrueNAS Scale is 35 days. I have set mine to monthly at the beginning of the month and every other month a long test in the middle of the month. Also a weekly short test.
The reason I am not running monthly long tests, is that my 20TB drives have an annual rating of 550 TB and I don’t want to get too close to that with my maintenance tasks (see my post referenced above for details).
In my setup, I use new NAS drives (WD Red Pro), so if you use very old drives/shucked drives/desktop drives, I’d run those tasks more often (or - in case of desktop drives - replace them with something more appropriate).
IIRC, long SMART tests take longer, because they also check unused blocks, whereas scrubs only check allocated blocks.
I’m not sure that adding more disks would mean longer durations for scrubs or long SMART tests, because I would expect those tasks to run on all disks in parallel. But my expectation might be wrong…
But even if those tasks were to run longer, what would the problem be with that? You can still use the pool, even though I’d expect performance to be somewhat degraded.