Thats a good question. First of all, consider yourself fortunate if you are Germany and Russia is about to fall. When I play Germany I will not spend any ipc$ on a navy in mid or late game until after Russia has already fallen and I have made sure there won’t be a successful amphibious assault on either Paris or Berlin. The reason for this is twofold, 1) the allies usually quit after moscow falls anyway so why delay the game by slowing down the assault? And since there is probably a sizable Allied force in GB or Norway, you must make sure that you are safe and 2) After moscow has fallen and if Germany is safe from attack, Germany should have sufficient cash to build a navy at its leisure in the Mediteranian. In fact. let Japan give the US trouble in Alaska/West coast so that USA diverts resources from the Atlantic theater. After Moscow falls usually time is on the side of the Axis, unless Japan is about to fall also.
If Moscow has fallen, and Germany and Europe are securely in Axis hands, then and only then would I build the German navy to retake Africa and if necessary start the invasion of GB or USA.