Ever few days, Windows Defender gets that yellow warning triangle, that a scan is needed.
I'm not getting these. The only time I see a warning that a scan is needed is when the computer has been turned off for several days and a scan has not been performed recently. Most notably I see this on my notebook because I don't use it often.
Microsoft Defender (formally Windows Defender) is designed to run when the computer is not being used or when you go idle. If you always turn off your computer when done using it, that may be part of the problem (though technically, telling you a scan is needed is NOT a problem). I just let mine go to sleep.
You can look in Task Scheduler to see when it is scheduled to scan. If no task is scheduled, it just scans based on the time it last scanned. If you set a schedule, it will scan at that time - unless you power off then it will attempt to scan when you come back.
My advice is to scan, then just let your computer go to sleep. Much happens to our computers when we are away - good things. Windows updates, housekeeping tasks (indexing, defragging and more) and our security programs (not just Defender) scan.
If you want to change the scan times and frequency, see
How To Set Your Own Scan Schedule For Windows Defender Antivirus and scroll down to "Schedule Updates With Task Scheduler" (don't ask me why it says "updates". I think he meant to say "Scan times").
For the record, I recommend once daily scans though weekly is probably sufficient since Microsoft Defender is already scanning everything in real-time anyway.