Maps are updated every 10 years.update it this year and you will be set. That said mapping software is updated every year by takeing info from other sorces. If the software has bells and the like you want upgrade more often.
I think Tom's answer needs a little expansion. Now remember, I'm pulling all of this from memory so it may not be totally accurate.
USGS 7.5 Quad appox. every 7 years via aerial mapping and comparison (At least that about the update cycle we had on our military maps)
Delorme, gives annual updates I believe.
Mapquest updates every quarter.
Google Earth updates as they become available.
So you can see, there isn't a cut and dry answer. The Tiger Files we use with GRLevelX software are updated with the Census but take their time becoming available.
As far as MS Maps and Trips, I would find out who the parent cartographer is. At one point I think they may have been Rand McNally. As to how detailed the map is? Again, depends on the company and source maps. Some will show private roads, some don't. I've seen a couple that would show a rabbit path as it dissapeared into a tree line.
Again, USGS will generally update their maps about every 7 years unless there are some really major changes (New High way or something like that) that will really alter something. Then they will issue and updated map wiht the new additions in purple (magenta?). The base maps are updated via aerial photography and changes made based on the comparison. I've seen a few Quads that haven't been updated in 30 years becuase tehre wasn't a significant change even though they aerial photograph had changed.