A young malesub into chastity asked about getting his girlfriend to lock him for more than an hour. My response:

The thing to watch out for is that after about two years, people's sexuality reverts to its baseline.

In other words, new relationship energy and sheer novelty will carry a partner well off their normal course, but only for a couple of years, then it snaps back.

So at this stage, it's really important to shape your kink offering to fit her natural inclinations, and make sure there's no overhead for her. An added complication is that partners with a strong dominant side tend to not want to go along with somebody else's script or do homework.

From what you've described, she likes teasing you, but maybe she isn't interested in the emotional labour of keyholding. The obvious solution is for you to just put the keys out of reach, or use a cheap timer safe. That way, she's not keeping you locked for longer, but rather enjoying the experience of you being locked.

It's also worth reviewing the practicality of your chastity device. She may have a shrewd suspicion it will create fuss and mess for longer sessions.