when is the best time to slip the punch inside?

I can't give you specifics but I think it depends on what you are trying to set up as a counter to the slip... it depends on what you think your opponent will be expecting based upon what you have thrown in the past and weaknesses you see in an opponents defense when he punches...

Like I said I can't give you any specifics but there is a lot of good info already on this board... I would recommend you read the following posts... it may not tell you when it is best... but it will tell you what options that you have available to you once you do slip to the inside...

CounterPunching: Right hand Counters to Right Lead
http://www.saddoboxing.com/boxing_fo...c,30823.0.html

CounterPunching: Left Hand Counters to Right Lead
http://www.saddoboxing.com/boxing_fo...c,30822.0.html

Left hand counters v. the jab
http://www.saddoboxing.com/boxing_fo...c,30702.0.html

Not sure if this helps you...
Blackvenom-2001