I should say the other day we let her out for 5 minutes, then the next 10 and today its was about 20 minutes - all with me watching her. She'll respond to her name while in the house, but the moment she gets outside I'm completely ignored
I would do this (but probably a bit longer to start, maybe 30 minutes?), but if you can, do it right before dinnertime. Then she has a reason to come home because she knows food happens at X time. (Or if it makes you feel better, do it before breakfast so that if she does stay out, at least it's daytime.)
As hollyberry said, though, it is a leap of faith. You'll have to trust that she knows the house, and knows you guys, and will want to come home regularly to see you.
FWIW, though, before my cat decided to be a chicken and stay indoors all the time (his current "exploring" consists of walking out onto the landing for 2 minutes), he left the house after breakfast each morning and came back like clockwork by dinnertime.
You could also put out some wet food to lure her back earlier (say after an hour or two initially).