Hi, there, Jedohara.
It kinda depends on the weather.
If room temp is not to warm you can keep them outside the fridge.
I do so, and my O finishes the box in a week or two.
Within that time hardly any worms turn into pupae.
If your room temp is warm, it's better to keep them in the fridge, or you'll have bugs in no time. (your o won't mind, though.

)
Not feeding the mealworms will make the turning proces into pupae slow down.
On the amount, you just have to try.
I give my one year old O 10 worms at a time and just continue until he's had enough.
I did use the 3 minute rule when my O was smaller, but nowadays he more quickly satisfied and 3 minutes is way too long now.