With having many cats with letting a kitten out first time always makes me nervous. However depending on the size of the cat should be around 4-6 months and depending on what time of year.
In winter months I try and keep my cats in. I recommend your cat is neutered and vaccinations done. A cat should be in for at least a couple of months when rehomed. So they know this is where they live and come back inside when you call them.
A cat needs to trust you and be settled. I never let a kitten out straight away I always walk around the garden a few times. We live in a fairly quiet street, some considerations need to happen if you are on a busy road or in a flat. I recommend considering keeping them as inhouse cats and give them plenty of cat objects to amuse them.
I take at least a week walking around with them and showing them to look left and right before crossing the road. Then I let them out for short periods of time. They become fascinated with new objects like butterflies, bees, and plants this is why it is important to keep an eye on them because a bee sting can be nasty.
When to let then out?
Gradually I let them out more and more until I am confident. Making sure they are okay and will come in. When looking at what others advise they point out that in one study was reported 75% of cats were returned safely to their homes, while 15% remained gone for good.
On the first few short outings, I let the kitten explore for a little while. And then either carry them back indoors or call them have the treats ready for them.
If you have a couple of kittens work with each kitten and make sure they are comfortable with coming in. A small cat could get snatched up or venture into an open van or car since they are at the acquisitive stage. They will want to venture out more and more cats can travel some distance.
I find the first few times they will be hesitant with having many cats. They follow and keep together and do not tend to go to strangers since most of them was saved when unwanted.