Hello,
It totally affects by the size of your database. The server itself is updated in 5-10 mins.
But then the database is exported from 8.1 and imported back to 8.4 version. So if your HDB file is 1 GB - it'll take around 30 mins to 1 hour. So you should check the hdb size of your database and count accordingly. We suggest to do update in the evening, so less interruption to employees, and if you're hosted and on shared server, that means you will get more resources from the shared server in the evening - quicker update.
Best Regards,
Hansaworld support