Typically the gap between riders is one minute until the last few when it kickes up to two.
Check when the race starts and figure 1 minute per rider. That will give you a reasonably safe time to get back. Use race start, not TV coverage start. You can get how many finished the last stage off the official TDF page. I would doubt there would be many overnight drops at this point.
I'm only going with history, anyone who knows specifics for this year is a better source.