The amount of time required to product a complete error/warning list for a large, broken C# solution is probably more than you'd like to spend waiting on a compile.  The number isn't going to be accurate as the number of errors increase anyways, since you can easily end up in situations where hundreds of errors are caused by a single off-by-one typo; conversely, a single typo (say, an unclosed multi-line comment for a simple example) could hide many errors.  Attempting to measure how long a solution will take to fix using this number is therefore useless.

But there is still a way to see what kind of mountain you have to climb -- use Resharper. Its Errors in Solution" window can be of some help with getting past the compiler limit. 

In general, Resharper is a very good tool when doing some brownfield recovery.  The number of warnings you'll see in the Code Issues window will probably require a trip to a therapist, since it not only covers compiler warnings, but hundreds of different potential problems (redundant casting, parentheses, forgetting to specify culture in string.IndexOf(), forgetting semicolons at the end of Javascript statements**, etc.) but you can sort the problems by type and fix them across your solution all at once.

Resharper isn't the only game on the block, either -- JustCode and CodeRush are both very good tools as well and are priced similarly to Resharper.

 

(** yes I know it's not strictly necessary for the code to run but some JS minifiers don't handle missing semicolons very well)