Depends on the mower you're using - if its just an ordinary electric hover mower with a rotary blade, or a non hover with a rotor blade, you just buy a new blade when its blunt or damaged, unless you want to put it in for a service every year, when they might do it if it needs it. If you can find someone to sharpen it, or you can do it yourself, fine, but in the UK, unless you sharpen it yourself, you won't find anyone to do it - replacement rotary blades for a flymo, for instance, are around a tenner, or £10, so not expensive, and I've only had to buy 2 blades in 30 years, mostly because they were either bent or badly chipped.
If you've got a cylinder mower, specially if its got an engine rather than being electric, its usual to have those serviced yearly by a lawn mower servicing company. The service may, or may not, include sharpening blades, that's something you'd have to ask them about. Most professional gardeners with large petrol mowers are quite capable of keeping their mowers functioning well, but not always including sharpening - if the mower is heavily used, it will be put it in for a full service yearly as a minimum. Groundkeepers are probably even more rigorous and are often capable of stripping down and replacing parts on their mowers, as well as sharpening.
As for how you know when the blades need sharpening, its impossible to predict - there's a big difference between a mower being used every day for 4 or 7 hours, and one that's only used once or twice a week. Damage may occur to the blades from stones or other objects hidden within the grass, and may cause the blades to be warped, chipped or simply blunt in parts.
If you're cutting grass all day long with a petrol cylinder mower, it'll likely need sharpening in spring and probably mid season. Otherwise, the easiest way to tell is by looking at the grass after you've finished - if the cut is uneven, then the blades need attention. If you're using a cylinder mower of any sort, inspect the top part of the cut grass - if it looks cleanly cut, then the blades are sharp, but if the ends look a bit chewed or tatty, then the blades need attention. This doesn't apply with rotary blades - they don't make a clean cut in the same way.