Inmates can write as often as they like. There is no limit on outgoing mail at any facility. As long as they have what they need to send a letter, they can write daily if they want to.
The supplies they need, stamps, envelopes, and paper, are available through the commissary. Stamps cost the same as they do on the outside, around $0.68 at current rates, and the paper and envelopes are priced affordably. All of it comes out of whatever money is on their books.
For inmates who have no money in their accounts, the facility provides these items at no charge under its indigent inmate program. Mail is treated as a protected and essential activity inside correctional facilities, and an inmate with no funds is not denied the ability to correspond with family. They may be limited to a certain number of stamps per week under the indigent program, but the access is there.
There is no reason an inmate cannot write regularly. Putting money on their books through InmateAid makes it easier for them to buy stamps whenever they need them, rather than depending on the limited indigent allotment.