You do not need to keep track of this yourself. You will always be invited for a follow-up, and you will receive that invitation by email or as a message.
You always receive an invitation
When it arrives depends on your diagnosis. For one condition it comes after a few weeks, for another only after a few months.
Your dermatologist decides that moment, because they know when there is something meaningful to assess. So you do not need to put anything in your diary.
You can always schedule one sooner
If you do not want to wait for that invitation, you can start a follow-up on your own initiative. You can do this at any time.
Good reasons to go earlier: your complaint is getting worse instead of better, you had side effects, or you are unsure whether you are applying the treatment correctly.
Your dermatologist is away
If you schedule your follow-up with your regular dermatologist and they are away at that time, you will see a notice about it. If you click "Okay, it can take a bit longer", you will wait for your dermatologist. If they become available again within the period you chose, your consultation will go to them.
If you do not want to wait, you can also schedule your follow-up with another Skindr dermatologist. Every dermatologist can see your file and your previous consultations.
Scheduling too early achieves little
Your skin needs time to respond. Allow about four weeks of treatment before there is anything to assess.
If you go earlier, there is a real chance that your dermatologist will not be able to say much yet. You would then pay for an assessment that mostly results in "give it more time".
If you are waiting for a blood test, schedule your follow-up only once you have the results. That way your dermatologist can assess your skin and your blood values in one go.
If the invitation arrives too early for you
If you started your medication later than planned, or you are still waiting for a blood test, the invitation may arrive earlier than is useful for you.
In that case you can simply leave it for a while. There is no deadline and nothing expires. You schedule your follow-up when it suits you.
If your complaint quickly gets worse
Then do not wait for your follow-up or for the invitation. Read "Can I ask medical questions by email?" for the right course of action, and what counts as urgent.