This question gets asked often as well. How do I know how much more it's going to stretch? It's difficult to say, really. But I can tell you that for the most part, leather can and does stretch. Especially when it come to motorcycle gear.
I recently acquired an older Rev'it pant (Marryls, pictured above) because I found them on eBay and couldn't pass them up. They're the BEST leather women's motorcycle pants I've ever seen on women of varied shapes/sizes. It's sad that Rev'it discontinued them here, because they're absolutely incredible. Of course, you can still get the fabulous Gear 2's but if you want something all leather without venting, these were a great option.
As soon as I tried to get them on, they were definitely difficult to zip up. Like, couldn't.zip.up.at.all. Like many of us do, I gave up, took them off and hoped I could drop 10lbs by the time I bought a motorcycle.
The problem was that they fit everywhere else SO damn good. I mean Perfect. I admit, I like my gear SNUG. I don't want a saggy butt, or saggy waist, or saggy anything. And that's generally how you want your gear to fit, if you can. It's a lot like clothes, when you wear bigger sizes you simply look bigger. Of course, it's also directly tied to safety as the placement of your body armor needs to be close to the body and immoveable in case of an accident.
I eventually slid them on and as soon as I pulled them over my hips, the stretching process started. Slowly I was able to pull the zipper up all the way (after 2-3 minutes or so) and it started to immediately feel better. You can see in the pic above that I could barely button the top, because the flap is showing. This is why it's SO important for you to try leathers on for MORE than 30 seconds. Of course, don't forget those crucial baselayers to make the process even smoother. Another 5 minutes later and the waist/hip started to feel more comfortable to where I could sit in a chair, lean forward and start typing. I was even able to button it on the tighter (inner) button at the waist.
I know they're not done, and I have more stretching to go. I'll probably get about ~half size of break in and then they'll be loose enough to hopefully wear my winter weight thermals. It's been almost an hour and they feel SO much better than when I first started.
So don't give up, just give it a few minutes.. maybe 30..