Legal Question in Civil Litigation in California

Well, first of all, they get to take your deposition once. If there are multiple defendants, and they were served with a copy of your deposition notice, and did not show up, they don't get to depose you later.

It is not clear whether you have had your deposition already taken, and they are taken it again, or this is one deposition that is going to be long. A deposition that continues from day to day is just one deposition, not multiple depositions, even though it takes place over several days.

Lengthy depositions are necessary, or at least reasonable, in some cases. The appropriate length of a given deposition will depend upon the allegations in the case and the witness's role in it. We have no information about your case, so there is no way we can say whether 15 days would be unreasonable.

Courts are generally willing to issue protective orders when one side demands too much in discovery, including an overly long deposition. If your lawyers think you could not win such a motion it is because they believe a lengthy depo is appropriate in your case.

Your question suggests that you are the plaintiff. If your allegations and/or the other parties' defenses rely upon a long and complex set of facts, then it may indeed be appropriate to question you about them at length. Since you are presumably the one who made these facts relevant to the case, you cannot complain about being made to testify about them.