How many batteries for Garmin Oregon 550 for 8 days?

Can anyone with Garmin Oregon 550 who has used it for recording track logs over several days tell me what kind of battery life they get with lithium batteries?
I usually only use my GPS for confirming locations and leave it turned off for the majority of the time so I've only ever used the re-chargeable batteries that come with it. I'm doing an 8 day walk soon and was thinking about using the GPS to make a track log via the Trip Computer function and want to figure out how many batteries I'll need. Will a set of lithium batteries last 2 days or 3? (I expect the GPS will be on for between 6 & 9 hours per day)
I've gone through and set everything to the most battery efficient settings (I think):
Tones, off
Display, backlight timeout, 15 seconds
Display, battery save, on
Heading, compass, off (not sure about this one, read somewhere that it saves batteries, will it affect the track log?)
Any other battery conserving tips welcome.
I usually only use my GPS for confirming locations and leave it turned off for the majority of the time so I've only ever used the re-chargeable batteries that come with it. I'm doing an 8 day walk soon and was thinking about using the GPS to make a track log via the Trip Computer function and want to figure out how many batteries I'll need. Will a set of lithium batteries last 2 days or 3? (I expect the GPS will be on for between 6 & 9 hours per day)
I've gone through and set everything to the most battery efficient settings (I think):
Tones, off
Display, backlight timeout, 15 seconds
Display, battery save, on
Heading, compass, off (not sure about this one, read somewhere that it saves batteries, will it affect the track log?)
Any other battery conserving tips welcome.