hi
if we have to run the engine for really long time,
more clearly if we have to take arnd 100 flights a day ,
what are the precautions for maintaining engine performane ?
how long the gaps shud be  ?

Run them real "rich".  If the backplate is cool while touching, I don't see why they can't run for really long durations.  Some components have shorter life spans (like glow plugs), but still should be within the range you are talking about.

Can a model take long flights or for that matter 100 flights a day?

It should be possible.  People have flown cross country many times before. 

Like I said before, keep your engine a bit rich, and make sure you have enough battery to drive your electronics.  Refuel as needed. I think the engine would like it if you wait a few minutes if you did shutdown, but even that may be a nice to have.

Wait for engine to cool down in between each runs, and with 100 flights a day even pilot concentration would be a problem unless you are talking of multiple pilots.

BTW considering that you would not fly during the night, there would be max 12 hours of light availble, that means 720 minutes, so even if your each flight is 5 minutes, 100 flights means 500 minutes which leaves just another two hours for you to do what ever else you want to do in between  

He can always fix some lights and do night flying