How can long you live with a tooth cavity before it causes serious complications?

Short as possible: There is no knowing how long or how deep it may until you see a dentist. It can progress very rapidly. It will only get bigger leading potentially to the need for a root canal or extraction. When early and small it can be fixed. Don't wait.

Depends : It depends upon where the cavity is located on your tooth, how deep it is, and how good your oral hygiene is. It's a good idea to have a dentist check it out.