There's not really a solid answer to this because people from all over the world use Tapas, so even if you post at an "ideal" time for one country or area, chances are it'll be at a really inconvenient time for many many other time zones.
Even just picking a day to post is highly subjective- some people would say "post on the weekends, more people have free time then" but then you'll hear other people say "well, I only read webcomics on the week days, during my commute to and from work" and so on. There's just no way to please everyone, in other words.
So you have a few options.
One is to target a specific demographic and target your uploads towards them. If you pick a specific country or time zone you can start making educated guesses about when your demographic might be getting on their app to read comics. What age group are they in? Do they have work/school? If so would they rather read in the morning, at lunch time, or afterwards? Is the weekend best? Again these will still differ person to person, but it's a starting point.
The other consideration that I've seen suggested but have yet to try myself is to find a premium series that is in a similar genre/content-matter as your comic and try to ride their coattails, uploading a short time after their scheduled release times. The logic here is that premium series are more heavily advertised and are generally are what draw casual readers to the site. So they'll be subscribed to those series and check in to catch the latest episode. If you're lucky, after finishing some of them might check to see what else there is to read and if your series is there when they look, it may warrant consideration. This seems like sound advice to me, tbh, and is something I'm considering for my upcoming series.
Best of luck!