FWIW, depending on where it is I am flying, I like to see the GPS data base updated at least every 15-18 months or so for VFR purposes.
A 15-18 month upgrade cycle to me is a nice point that works out to about 3 newly issued sectional charts for most of the country. Since most of my flights are semi-local (<300NM) I try to time it with the local sectional... Of course where I am is in the overlap between Washington's Sectional (31 Jan-18 Jul) and New York's sectional (25 Apr-7 Nov) so one area is almost always more outdated than the other.
If there are major changes to the airspace in my area (especially the Bravo's) I'll update it more frequently. I'll also update it more frequently if I expect my flying in the next 6-12 months to be mostly in unfamiliar areas further outside the local area. Beyond that, airspace and waypoint changes for the occasional trip out of the area is usually covered by my GPS iPad with Foreflight or iFlyGPS.
Ultimately it depends on the type of flying you do. If you never leave your local sectional chart then you can probably do an 18 month schedule, sync'd to your sectional chart issuance dates + 1 month, pretty easily since you are quite familiar with the area and it maximizes the utility of the update providing the updates of 3 sectional charts in 1 update.
If you frequently fly outside your local sectional and into unfamiliar territory, especially since neighboring sectionals rarely align date wise, it would make more sense to split the difference and go with a 15 months schedule.
Few of us regularly fly VFR further than a neighboring sectional away but if you do, something closer to 12 months schedule might make sense for you.
I dont see much value in anything lower than 12 months unless there is a major change you are aware of (such as a redrawn Bravo) that you definitely want to have in your update, otherwise updating every 10 or 11 months (or sooner) is not all that different than updating every 6 months for the VFR pilot.
As an aside, I update the GPS in my car every 12 or 24 months but automobile GPS updates dont get regular incremental updates throughout the year like aviation ones do... In that sense, if I update at month 18, my maps are already 6 months out of the yearly issuance date and only have 6 months of utility left to them so if I forget to do the update right away, I usually wait until the next issuance date... This isn't really a problem with Aviation updates that are done every 45 days but is a factor worth considering if you are looking at getting an update within a week or 2 of the next IFR chart release.
