nope, don't tie a string, the stars and crabs are hand fed, and the shrimp will steal a piece of shrimp from a hermit.
I take the shrimp out, put it in a cup of tank water, take out the brine, feed the clowns, do a count of all inhabitants. then cut the shrimp into three pieces, two go to the stars, the other gets cut up more for the hermits, then i feed. i'd say the shrimp is out b/w 5-10 min, depending on what else i'm doing, before being fed to the stars/hermits.
if you meant how long do i leave the shrimp in the tank, i don't take it out. there is never anything left to take out. one cocktail shrimp, cut in 3 pieces, 1 of the three cut in about 6 pieces...doesn't leave much left over, the stars eat thier piece, the hermits eat the rest, and if by chance a piece falls...the shrimp will eat it or other crabs will come out and eat it or the stars will find it.
btw, i hand the pieces of shrimp to the hermits (the shrimp is the only one who won't eat from my hand...he likes to steal it from a hermit or star).
don't put more shrimp then your clean up crew can eat...if you have to only feed half a shrimp.
hth