There is no one way of knowing when to seek professional help or support for sexual problems. The right time to do that is whenever is right for you.
If you are single and feel there are specific sexual concerns or issues that you can't figure out on your own or work through with the support of friends or family, then trying to work with a sex therapist can be a helpful new way to approach the issues.
If you are in a relationship the decision about when to see a sex therapist might be a bit more complicated. Does your partner also feel that seeing a sex therapist is a positive step in resolving sexual issues or concerns? Are you planning on going together, or are you interested in going on your own?
An ethical sex therapist will suggest having an initial consultation, and if they feel that sex therapy isnt going to be helpful they will, or should, let you know.
Sex therapy isnt necessarily for crisis management (although it might do that as well) and even if there are parts of your sexual relationship you are happy with, if you feel that you could benefit from some support, education, information, or counseling from a professional who is trained in the area of human sexuality, then exploring sex therapy as an option is perfectly reasonable.