My experience with this personally and after speaking to friends is if you don't introduce a teat before 6-8 weeks then there is a good chance you will struggle to get your baby to accept one at all.
I have 3 friends who cannot get their baby to drink expressed milk from a bottle. And it was all because they tried to introduce it too late.
I would also say, that if you start giving a bottle, give it regularly. As I had a baby that forgot how to use a teat and it took a stressful few weeks and lots of wasted breastmilk to get her to take it again after we didn't bother for a while.