I’m assuming you don’t want to inactivate accounts, but want to change what this preference setting considers to be inactive
When I don’t visit here, send me an email summary of popular topics and replies
AFAIK the time is one week. I’m here often so it doesn’t apply to me, but if it did apply I might turn that off if once a week felt to be too frequent for me and it felt more like a nag than a useful summary of what I was missing out on.
I understand that you want members to actively participate but I’m not so sure that sending more emails is this best way to achieve what you want.