I have the 7.4V version of that and have tested it around 5 hours...so you should be able to get a full show out of that unit...but if it was me, I would just spend a little extra and get the 7.4V...a little extra capacity cant hurt...
I would also recommend a dual batt charger:
http://buyandmore.com/details.cfm?modelno=BCD-641&cat=ignorealso comes with the cigarette lighter connection...always nice to be able to charge a couple at once!