What is the typical delay relative to to the maximum delay?
There is no rule-of-thumb that I am aware of.

First consider what causes variations in parameters:

1. Temperature
2. Voltage
3. Factory Processing
4. Aging


For #1 and #2, if your parts will see a smaller variation than what the datasheet indicates, you can take credit for that.

For #3 and #4, these can often be considered to be random, it is extremely unlikely that all parts on your board will be manufactured with worst case processing, or drift in the same direction with age. The methods to account for these are tedious: RSS (Root-Sum-Squared) analysis or Monte Carlo analysis. A lot of engineering is fun for me, RSS is not.

If you don't need to prove worst case by analysis (company policy or customer), and there are only a few critical paths, then consider measuring the paths on the boards.