Hey arequimis,
Unfortunately having dynamic service duration is not something that is possible out of a box, at least in the free version of the plugin.
There are couple of workarounds that would allow you to have something like this but each of those would require multiple add-ons that are available only in premium version of the plugin. What you could do is either having different services for each of those durations, this means having separate service for 1h service, 2h service and so on.
Aside from that if you’d want to have that as one service, you could use paddings and add additional time per service providers, both cases would require you having Durations add-on and the second one would require Paddings add-on.
Luckily we do have a free 14 days trial currently available so you can sign up, grab Appointments+ for free and see if this could work for you ??
Cheers,
Bojan