Thumb Sucker

Who out there has a thumb sucker? My daughter never took a pacifier but has sucked her thumb almost since birth. At first it started as a cue she was either hungry or tired. Then she started sucking it to get herself to sleep. Now it's kind of just all throughout the day unless she's being very active outside. But it seems if we're playing inside it's always in her mouth.

When should I be concerned that she's still sucking her thumb?

How do I help her stop when that time comes?

My niece was a thumb sucker and she sucked in until she was 5 or 6. I'm nervous my kiddo will do the same if I don't know when/how to help her stop.