What time can be considered enough to withdraw my submission?
The review process often takes a long time. 2-3 months is typical. I’ve known instances of it taking 9-12 months. You need to be patient. This process is about the same for each journal (well most take a month or so at least. You must consider resubmitting to another journal carefully as you won’t save time by starting over, even if you don’t need to reformat it or write for a different audience (note that you cannot submit the paper to more than one journal at a time).

That it’s still with the editor (hasn’t been rejected) is a good sign. If they believe your manuscript does not have relevance to their journal or sufficient merit, they would have rejected it by now. They will now only do so if the reviewers raise serious concerns, it’s often accepted with revisions if it goes out for review.

The editorial process takes a long time. Many scientific or academic editors perform this role as a part-time job on top of their own academic responsibilities. The most time consuming part of the review process is finding the reviewers. They need to find reviewers with the right expertise who don’t have an existing relationship with the authors who are available to review. It can take a long time to get responses for requests to review.

Once they find reviewers, the review process should be quick. Reviewers are only given a couple of weeks to return their completed review (usually they submit them in this time or a few days late). They should decline to review if they cannot do in this timeframe. Once the reviews have been returned, an editor should give you a decision and the reviews within a few days.