Depending on the tool you’re running, a scan can take between a few minutes and several hours.
It may appear that a scan is stuck or frozen, but you can click on the target name to check the progress bar info tool-tip to see what test is being performed.
The scanning speed also depends on the response time of the server and the network latency on the server-side. It will finish eventually.
If any of your scans are running for more than 24h, please contact support. Your scan is probably frozen and you won’t be able to run new scans if you hit the maximum parallel scans limit.