I'd say it's better not to rely on one fixed schedule, since maintenance needs can vary depending on the tank size, household usage, and overall condition of the system. Slow drains, unusual smells, wet areas around the drain field, or backups can all be reasons to have things checked.
Septic.ca is also a useful resource for learning about septic pumping, inspections, and general maintenance. Keeping basic service records can make it easier to stay on top of future maintenance.