When is the right time to start
adding(or thinking about adding)
servers to your Web Application?
When you are designing your application. I've seen too many applications not designed to use multiple servers, and reverse engineering that later can be horrendous.
Make sure you test on multiple servers, too. Again. I've seen many applications work fine in dev/test only to fail in production because they couldn't deal with load balancers, or firewalls, multicasting hadn't been considered etc, etc.
And the time to add another server is when your capacity management statistics suggest that you will run out of capacity in just over the time it will take you to add another server.
You are collecting cpacity statistics? No? Then another thing to talk about with your application developers and infrastructure management people.
I disagree that waiting until you actually get 'server too busy' and irritating your users is the correct thing to do. Adding a new server to a production environment can be a lengthy process, and waiting until you have errors before you start is not really wise.