This is going to be entirely dependant on the anti-virus tool you're using and the environment it's deployed.
Here are some characteristics to consider;
- Time between signature updates
- Impact on system usability during a scan
- Capability of your AV tool
- Endpoints running your AV tool
For mainstream corporate AV solutions, it's typical to see signature updates every 24 hours. This may mean you want to run a scan of some kind on your systems as soon as they've been updated to detect new malware: how up-to-date is your detection engine?
If you have any legacy systems or are running intensive applications, you may want to wait until the weekend to run a full system scan: what is the impact of running a full scan on a system?
The capability of an AV solution will vary. Some may only offer cursory protection when a file is ran, others may offer granular detail whereby policies can be implemented to scan newly created files or recently modified files: what is your AV capable of doing?
Not all endpoints require scanning at the same frequency. Some devices may be rarely used or are covered by other protections, thus reducing their potential for infection, others may be deemed high-risk and will want to be scanned every day: What are your low/high risk assets?
There is no real right or wrong answer here but as a security-minded individual you'll lean towards as frequent as possible but ensure you are aware of the functional impacts: consult your users, admins and assess your environment. In an ideal situation you would be running scans constantly but in reality, it's just not possible.