From the info from our backup file - it takes little time for the incrementals to complete. Our incremental is up to 193M nightly. Here is our info..
Label: incr-20070314.050005.110
Type: incremental
Status: completed
Started: Wed Mar 14 01:00:05 EDT 2007
Ended: Wed Mar 14 01:00:34 EDT 2007
Redo log sequence range: 78 .. 78
Number of accounts: 81
Label: incr-20070313.050005.506
Type: incremental
Status: completed
Started: Tue Mar 13 01:00:05 EDT 2007
Ended: Tue Mar 13 01:00:22 EDT 2007
Redo log sequence range: 76 .. 76
Number of accounts: 81
We run our backup at 3:14 and its done in less than an hour. Well before any employees come in to office.