This is really up to personal taste!
A great way to indicate as to when to start tasting your spirit is by looking at the colour. When the colour matches the approximate colour of bourbon you should begin tasting your spirit.
If you are using the Air Still or the T500 then after the staves have been filtered out, add the flavour and wait 24 hours to taste.
Should you use too many Oak Staves, you will need to take the spirit off the wood and allow it to age and eventually the flavour will drop away to where you want it.