From an answer on Quora by Jonathan Klein, 12ms for a 1500 line .htaccess file:
Having a large .htaccess does have a cost. Ours is currently ~1500 lines and we benchmarked the time spent parsing it at around 10-12ms on a production webserver. Hardware makes a difference obviously, but you can fairly safely assume that the cost of that 3000 line .htaccess is around 25-35ms per request.