It does depend on what type of haylage you are using. However, as a rough rule I would feed the same amount of haylage by VOLUME (however, it will WEIGH more than a similar amount of hay because it contains more water). Haylage can vary a lot in energy levels depending on the time it was cut and the grasses used. If you are using a high energy haylage and the horses are getting fat, then cut right back on hard feed (if used) or find a lower energy haylage.
i know they say you are supposed to but mine dont seem to eat that much of it. i just give them what they are able to eat for the night and they always have some left over. probably works out about the same amount as hay.