​What do you consider “late”?

I have an office with 4 people. Half start at 9a and the other half starts at 12p. All 8 hour shifts. We have a very relaxed office environment but I’ve found recently that the relaxed environment is causing a lack of productivity. I don’t want to micromanage, but one of the first steps to try to curb this lack of productivity (which takes a noticeable toll on the business), is to firm up that you must be on time. 3/4 of the employees consistently walk in the door 8-20 mins late and/or leave 5-10 mins early without saying a word to me. In fact, I had an incident where one walked out the door 15 mins early while I was on a call in my truck in the parking lot, walked past me, waved to me, and left.

What is a reasonable amount of time after start of shift to consider late?