From what I've seen, when professor do accept students from their own labs, it's because the student has been in the lab long enough to grow and prove that they are qualified and that they are a good fit for the lab and the program. Because it usually takes time for professors to get to know new RAs and trust them with upper-level research projects through which they get the opportunity to prove themselves as researchers and writers, this approach would likely be much less effective if you were there for less than 1.5-2 years before applying. However, it really does depend on the faculty. Some faculty never accept from within and encourage their RAs to diversify their experience; others are totally willing to hire from within if they have an RA with a really good fit.