The best way to get the number of moves for every game in a database is from the game list. There is a column with title "Moves" which gives you the game length.
You can sort the display with a click on the Tab.
Furthermore there is a second column "Line moves" which shows the number of moves in all variations. Normally this column is hidden.
You can open it by right clicking in the list header and choose "Line Moves".
