Battery replace time?

During GUADEC I decided that I need to buy new battery for my Dell D400 laptop. Current one is giving me 40-50 minutes of working only. According to ACPI informations it is really low charged:

11:44 hrw@maluch:BAT0$ cat info
present:                 yes
design capacity:         42000 mWh
last full capacity:      11530 mWh
battery technology:      rechargeable

So it does not get even half of nominal charge. I wonder how much time I will be able to get from 4400mAh battery (current one is 3800mAh when most of available ones are 3600mAh).

And this time I really need to buy one.

UPDATE: bought one — will get it in few days.