I want to know in Detail when we can go for automated tools instead of manual testing
Please answer me.
Whenever your regression tests show pass percentage more than 95% and any major blockers or operational defects are not present in product you should attempt automated testing. Don't try automation alongwith RAD (Rapid Application Development) approach.
In net, you can fine one article "When to Automate" by Brian Marick... It will give some more insight to u.