system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek]  How exclusive are the holodecks?  Do only officers get to use them most of the time?

It seems like there are never many on ships that have hundreds or thousands of crew members.  Or when they say "holodeck" do they literally mean there is a deck consisting of only holo-rooms, in which case it might be a bit better.
 
Also I know they do tend to book time slots in them.  But still.
gpt
Well, during the mid 24th century, holodeck technology started to mature and come down in price. It was found on more Vacationy planets and more matured and industrialized planets first. There were tons of them on Earth and Riza before they became standard on starfleet ships. 

One of the great boons for serving on the Galaxy class ship, and its counsin class the Nebula, is that they were among the first ships to have holodecks as standard compliments. 

And during the later half of the 24th century and first quarter of the 25th, Starfleet has a retrofitting program to get all their ships that meet their requirement to have them installed, as well as an extensive program to get them installed on all major starfleet facilities. 

Now, as for restriction. Its a free society. As long as you're not on duty or under a demerit, you're free to use the holodeck, but holodecks can only accommodate so many so there is a reservation system in place that handle generally by the Operation Officer, with Official Business and seniority, and rank coming into play for who gets what time slots and how long. So while no coms and first year ensigns might not get the times most convenient for them... they still all get some time on the holodeck.