Your mother may want the best for you, but she does not understand the importance of boundaries or the difference between talking to a professional and a mother. A therapist is not a friend or a family member, and that’s important, because with friends and family members you do have to consider what you say to them and consider their feelings. What if the issue is with them or something that causes you shame? Aside from being an outside non-judgmental professional, their entire job is to help you understand you. It would be an inappropriate burden to put on anyone else, whether or not they invite it.
I suspect that you have an enmeshed relationship with your mother where there is some degree of codependent, Does your mother expect you to talk to her about your issues with her? (This may be a fear she has. It’s rather common for caregivers, particularly mothers, to fear their children going to therapy and speaking ill of them. It might help you reassure her that you love her when discussing your interest in seeing a therapist.)
To answer your question, the time to see a therapist, as opposed to seeking support from your mother, is when you feel it’s important to do so.