What you have written is correct. You may also want to add a reason for it being a waste of time. Other ways of saying it would be - It is not worth the time. It is not worth the effort Don't give yourself to a lost cause. It is a lost cause.

When you say that something is "a waste of time" then it means that the speaker thinks the thing isn't worth doing at all. Personally, the phrase I would use for what you want from what I understand of your questions is "don't spend too long/much time on this". Hope this helps!