I believe that the limit to any expression in a query is 255 characters. Nesting in IIF is fairly straight-forward (although it can make your eyes go bleary).
IIF( condition1, IIF( condition2, IIF( condition2b, answer, answer), IIF( condition2c, answer, answer) ), IIF( condition3, answer, answer) )
and so on ...
Just get those pesky parentheses and commas right!