Any grass hay is fine to feed as often as you want (timothy, orchard, mountain grass, bermuda, brome, etc.).
If you have a pet chin, I would not give unlimited alfalfa. It is fine as a treat or to add some variety but if your chin is an adult and not being used for breeding, unlimited alfalfa can lead to kidney stones as it is much higher in calcium.
Grain hays such as oat, barley, wheat, etc. also should not be given in unlimited amounts. These hays are rich in phosphorous (and low in calcium) and feeding an excess amount of this long-term can cause calcium deficiencies. It is fine to give as a treat in addition to the grass hays though as it is very coarse and really helps with grinding down those molars.
I find that giving my chins a variety of hays seems to keep their interest in their hay. They all have different textures, flavors, and benefits. Personally, I use timothy as a staple. I give alfalfa as a treat and also give it to any young/growing chins, and pregnant/lactating females. All the other hays I have (oat, wheat, barley, and orchard right now), I just blend in multiple times a week for variety - this is most important for my dental chinnie, Rayna. Since I started giving her a variety of hays and supplementing with vitamin C, she hasn't had to go back for a filing in over a year (the vet said her teeth still looked great at her appointment last month)!