I have been using dildos on my wife to introduce her to the idea that bigger cocks are better. She is new to bigger cocks, but now admits she likes them because she she cums way harder. The issue is she is so tight and it's a bit of a challenge to use our thicker toys even with a lot of lube. The dildos are big, but not overly huge (6" to 6.25" circumference, all 7" to 9" length). Will regular use eventually loosen her up to where this size thickness is just normal for her? How often and long will it take?
Yes and no - with regular use, it will loosen and become easy for her to take them, however, between each session, it will begin to contract back. I think the first time you try to get a larger size dildo in, it is the most challenging, once she's taken a certain size, I think it is easier to get back to that size. Part of it is psychological too - if she's intimidated by the size, she will have trouble relaxing, which will make things much more difficult.
Back before kids, I got my wife to the point where I could fist her and slip my cock in, or she could take huge dildos and she loved it. After we had a kid, which obviously is the ultimate stretch, we had to wait some time before resuming activities, plus having a baby made sex less regular. It ended up that her pussy tightened and, while definitely looser than when we met, I couldn't fist her anymore. With additional pregnancies, I've never had the time to work her back up to that level and so odd as it may sound, after 3 kids, she is now tighter than before we had children.
A good way to loosen things up is, during sex, slip in a couple fingers with your cock. My wife always enjoys that. Eventually you can add in a dildo with your cock and ask if she likes getting DPed.