Depends on the size, voltage rating of the capacitor and quality of the capacitor, but lower esr is better. I printed out this scale as a guideline, and find it useful.
http://www.electro-tech-online.com/a...003-jpg.84737/
It is also useful to compare readings to those taken on a new good quality capacitor of the same size and voltage rating. On large capacitors, say under one tenth of an ohm or lower, you may need to zero out or just subtract the test lead resistance to get a better reading.
jr