I find Puran Defrag to be very good, fast, configurable and also has an option to do a defrag on reboot.
I also have Defraggler, but I haven't used it since I installed the free Puran Defrag application.
The defrag duration tends to be related to your use of the computer some are constantly installing and uninstalling programs, downloading all sorts, so the drive can become more fragmented. So it would rather depend on how much fragmentation (%) is found on these few monthly defrag scans.
I do mine once a month using puran and one of the options it to optimise the folders to help speed application loading, etc. I could probably extend that duration to every two months, but on a standard hard disk this frequency isn't really an issue. On faster systems with fast NTFS SATA HDDs fragmentation isn't such an issue as it used to be.
Only if you have one of the new SSD (Solid State Disk) drives should you not defrag.