Hops are a bit like mustard... a little bit goes a long way!
I use 3g/litre (0.25oz per gallon). This is about 2 teaspoons per gallon in secondary just before bottling for 3-7 days. The cider takes on the hop flavour quite quickly so it is a case of taste, taste, taste frequently and pull them out when the flavour profile is what you want. I found that a higher concentration of hops simply releases the flavour too fast to finesse the taste, i.e. two days and BOOM it is too late (too "hoppy"). So like many things with cider, low and slow is the way to go.
Something that you need to be aware of is that the pellet hops in the bag will float, so I attach several large stainless steel nuts or a length of stainless steel rod to the bag and drop it all in on a string so that I can jiggle it (and get it out easily). I had been looking for a hops "torpedo" but they all seem to be too large to fit through a 1 gallon carboy neck. I did try marbles or nuts in the bag but they tend to bunch up and make it hard to get the bag out through the neck of the carboy because by then the pellets have disintegrated and you end up with a mushy hops and nuts glug in the bag.
A slight hoppy flavour makes a really nice cider IMO.