When I clicked on your question, I thought maybe you were talking about bottles, like wine bottles, in which case my answer would be "about one night around here!" ;)
BUT, you weren't! I would not reuse a bottle that is 6 years old. Plastics break down over time and it's not worth the small cost of new bottles to take a risk. I tried the fancy bottles like the Avent for my first child. Quickly found out she could care less if it was an Avent or a cheap old Gerber. I was and still am a stickler for sanitation, and found that only having a few Avents on hand was not going to work for me. I like to use a bottle, rinse it, and put into dishwasher. And I was not about to run my dishwasher 2x a day or buy a ton of Avents. Especially working FT outside the house! So I bought about 18 of the Gerbers and washed/rotated them. Came in really handy to make up multiple bottles ahead of time too. I also didn't feel so bad about buying enough of the proper flow nipples to match the bottles. And when we went places over night that didn't have kitchen facilities, I again didn't have to worry about having proper sanitation for our bottles.
Congratulations and best of luck to you!!