We serviced every hydrant once a year, that included removing the port & steamer caps, inspecting them, flushing the hydrant (full-open, many turns) some oil on the gland nut, and painting them if they needed it. Yes, colors show gallons per minute flow rates.
The water dept used to do the maintenance, but when the fire/rescue crews do their own work, the engine company guys get to know their territory a bit better, and it gives them a chance to get out of the station, as 87% of the calls in my jurisdiction were for EMS calls. We ran cross-trained firefighters/EMS workers, since 1975, and advanced life support since 1976. I've been retired for awhile and was one of the first state-licensed paramedics in Florida, I have a two-digit license number, they're over halfway to seven figures now, as the numbering is cumulative.
Sometimes someone hits a hydrant, and takes-off, unless the hydrant leaks, you may not be aware of the vehicle impact, which could result in a dry hydrant just when you need one. Depending on the distance between hydrants, you might need another load of hose laid to you to establish a water supply beyond the 300 gallons most quads or quints carry these days.
We also used to cut the chains on the port caps to make it easier to remove the caps.