Immediately the relationship starts, if you are really serious about him or her, you are already thinking about taking this person to your parents. However, the timing is confusing for a lot of people, but it all depends on you.
For some people, they do it when they are sure he or she is the one. For others, there are no rules. They will introduce you even when they are not sure it will end up in marriage. Just be sure it is the right thing to do at that particular time.