> Could you please tell me how long the daily maintenance usually lasts?
It depends, and it has different phases. There is a phase when the server slows down to complete backups (about 2-15 minutes). And then there's a phase when it may (or may not) reboot to enable security updates (usually less than 2 minutes).
> how can I upgrade to using the pro server s2survey.net and what are the costs involved?
After buying a licence in the shop, you can create a new project on the pro server and simply copy your previoud project there: https://www.soscisurvey.de/help/doku.php/en:general:copy-project
The costs depent on the survey administration period, and the sample size: https://www.soscisurvey.de/en/pricing
If you like, we can add a dummy licence to your account (please write an email to firstname.lastname@example.org for that), so you can test the pro server. This may also reboot, but the "slowdown" due to backups is much shorter (~ 1 minute or less).