At the age your piggies will be, they dont need alfalfa at all. It isnt a grass hay. It is only really needed for pregnant sows and babies when the are very tiny and before weaning age. After they are weaned at 3 weeks of age, they no longer need alfalfa. They can be fed a normal adult diet of unlimited grass hay, 50g of veg and one tablespoon of pellets.
You must ensure they have a constant supply of grass hay available. 80% of what they eat in a day needs to be hay and they are grazers so it needs to be available in large amounts constantly. Whether you do it by throwing in a handful or two regularly throughout the day or putting in large piles a couple of times (if you arent home during the day to top it up) is up to you but they must never run out.
The only way you will know they are getting enough hay is by weighing your piggies every week. If they gain weight well, then they are getting enough. You cannot judge they are eating enough simply by seeing them eat hay though.
Any significant weight drops (beyond fluctuation) means their hay intake has reduced and that requires you to step in with medical attention.
They most likely will never eat an entire pile though - they will pick through at the bits they like and leave the rest, so even if there is still hay left, its important you top it up with fresh a few times a day.
The amount you get through each week/month is going to vary on your set up - some are bedded on hay so you will use more, indoors/outdoors etc. People who have indoor piggies and only use hay for eating will use a few kilos every couple of weeks. Mine live in a shed and I also have five animals (two piggies and three rabbits) so my hay usage is huge - 20kg every 3-4 weeks.
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Mine eat lots of hay. I use a cheap meadow hay for the bedding which they will eat at times and use Haybox Timothy Hay as feeding hay which I normally put on top of various places and try to keep the meadow at the bottom as much as I can. There have been times when the piggies have eaten every last strand of Timothy despite putting lots in at a time especially this time of year. It is deceiving as they have lots of meadow so my boyfriend sometimes leaves it thinking they've got plenty of meadow on the floor (to get out of going to the garage!) But in reality they've got no feeding hay and they are hungry! So be careful with replacing it as much as possible and finding a hay that they will eat and not just tolerate. Good luck 
