it depends on the group
but there would be very few groups
who ride 100 miles with no breaks
how and where you stop
depends on where you are riding
if there is a bench
sit on the bench
otherwise
stretch and take a drink and a bite
and keep going
if you are riding alone
as i often do
stop whenever the hell you feel like it
the only things that limits the number of breaks i take
are mosqitoes and black flies