Secondary concept !?

[root]

Good  day ;
Maybe so easy question!

I have  3  dns servers running for many domains;

Lets say  one of the DNS's goes down or do not serve for a specific domain (due to an error on it's domain definition file;

for that  specific domain) how long it will take that the queries being redirected to other servers (next DNS).?

I've recognized that if the first DNS goes down the domain name is not being resolved by the second one!

but it if do nslookup by the second server it resolves the domain!

Thanks

ALbert