This topic is so individual - it really does need to be what feels right for you.
I started expressing and getting DH to give DS a bottle at 6 weeks. I don't know about "a magic window", but just went on the MW advice to get BF established before contemplating anything else. Also, I waited for 6 weeks because, for me, it took a while to get the hang of breast feeding. That was something I didn't expect - I thought BF was going to be a doddle - it is wonderful, but it took a few weeks for both myself and my son to "get the hang of it".
DS didn't take to the bottle the first time round (he guzzled it all and then threw up everywhere), but he did manage it on the second attempt. I then breastfed and bottle fed (both expressed milk and formula) for six months, which gave us quite a lot of flexibility. I have a good friend who only breast fed with no bottles at all, and that worked really well for her too. So, again, it's what works for you.
Oh, if you do decide to express, I found that I only got 1/2 ounce to start with, then 1 ounce, then 2, until finally a few months down the track I was getting 6 or 7. It seems it takes a while for the boobs to know what to do with the pump, so don't get disheartened if you do try, and you don't get much out to begin with.
Good luck SANA - look forward to chating with you about your experiences. Above all, if there is any advice anywhere that was useful to me, it was - DO WHAT FEELS RIGHT FOR YOU (regardless)