There is no rule. Good practice would be to get on the site and check frequently. If you're signed up for "My Garmin" and your unit(s) is(are) registered, the site will tell you when an update is available. I check for updates every month or two - of course I have a variety of maps installed on my 2 units and they aren't all updated at the same time.
Garmin updates maps on their website quarterly. I am on my third Garmin unit over a six year period. The only updates I have ever bothered to install were the free ones that I got with each new unit -- never had any issues.
The quarterly map updates are worth the cost, as freeways are being opened in different areas all the time and it is very east to be sent the wrong/long way when the GPS does not know about the road changes. The lifetime updates are worth every cent seeing as you get 4 per year.
The cost is approx the price plus 35% of one years update if done each year.
I suppose it all depends on how much your Garmin cost. Having paid just over £100 for my 1340, I don't feel too disposed to pay practically the same again for a 'lifetime of the unit' update, when the unit is only warrantied for one year, and could die on day 366. Almost as cheap to get a new one every year, and have it warrantied. Still at least it's not as bad as colour printers, where the first ink renewal costs more than the printer!
Register in MyGarmin, then go to My dashboard. This will show you what updates are available and let you download them. Check MY Maps and it will do the same for your maps. You will get the option of buying an update, or if you get Lifetime Maps (well worth it!) you can just downlaod them from there.
Garmin maps in Australia are updated every three months. I have read somewhere that maps change about 15% a year. You need updated maps if you travel anywhere where you are not familiar with. If you need to buy new maps you should compare their cost with the cost of buying a new unit. A new unit would of course have the latest maps as well as the latest technology. As with most things new technology (eg, advance lane guidance, spoken street names etc) fairly quickly filters down to the bargain units.
Does somebody know if there is any chance to contribute a change to the map (Europe)? I have:
Cnew Dem Europe DEM
CN Europe NTU 2012.20 3D
CN Europe NTU 2012.20 ALL trafficTrends Europe 4.25
I just live on the street in a new area for 4 years but the map does not have this street so nobody can find the way here, especially annoying taxi drivers say "you don't exist". Last time was funny: a lady was delivering the delayed baggage and kept telling me that she could not find my address on the navigator so she was about to give up.
NO!! sorry but untill gps mapping software catches up with all new surrounding area's and street it may take up too 18 months as I have found in my new suburban but then I have seen new roadways placed on the maps an not been constructed for 12 months. I think it's just the luck of the draw if it makes it's way to gps mapping updates.
The accuracy of maps would depend on the supplier of the maps that Garmin use.
In Australia it is Sensis, and I have found that the map updates each 3 months are very close to being accurate for altered or changed roads.