Feasible TTL.

[Salman Ahmed Hashmi.]

Hi guys,
Can anybody suggest a feasible TTL value for each of the following ?
This question may look ridiculous but i have been thinking over it and
even found that it is up to the administrator either to decide a default
value for all of his domains or give an individual value to each and
every domain.
The format is given below.
                                
                                900531422       ;Serial
                                3600            ; Refresh every 1 hour
                                900             ; Retry 15 minutes
                                2592000         ; Expire 30 days
                                3600 )          ; Minimum TTL of 1 hour

Furthermore While defining ttl values for a domain,
1)Is there any relation b/w the available bandwidth (i.e link capacity
)to nslookups ?....
And,
2)How can an adminstrator know which domain needs a faster update and
which domain is comparatively dormant.

Regards