haylage can be wetter or more dry ...that will alter how long it keeps once opened. The large round bales I get are fine and ok at about a week after opening . I do only pull back enough plastic to get what I want though , and then cover it up again afterwards. Try not to fluff any that will be kept for another feed as this will allow air in , and that will start the heating up process , that converts organic matter to compost . You could pop them in a larger plastic bag and tie it up once opened to slow down any bacterial action , but I find that by the time anything is starting to happen (occasionally ) , I am finishing the bale anyway .
As for how much ? well sheep keepers will tell you more than I can there ...I feed my haylage to horses more or less ad lib . You get to know how much they will eat per day , and can then judge by condition if you are giving either enough or too much.
Hope that is of some help ..
cheers
Russ