3-4 days. Reproducible since I optimized my N900.

I use power kernel v47 with undervolting.

Most important parameters from kernel-config:

0:25,90 250:33,180 500:43,360 550:49,400 600:55,430
Smartreflex VDD1 and VDD2 = 1
Ignore nice load = 1

I have a small script I call manually after boot to adjust several things:

# deprioritize system background tasks
renice 15 `cat /syspart/applications/standby/background/tasks`
renice 15 `ps | grep modest | cut -c1-5`

echo -n "50" > /proc/sys/vm/swappiness
echo -n "7" > /proc/sys/vm/page-cluster
echo -n "1" > /proc/sys/vm/laptop_mode

and some other minor modifications (see swappolube, I just set these values manually)

I use the Wifi-Switcher app to load/unload the wifi driver whenever I switch wifi on/off. I switch Wifi on only when I need it. The same for bluetooth.
Most of the time, I am in standby GSM.
Display brightness at 2. I switched off display vibration feedback, most LED feedbacks (except charging battery)

I have a class 10 microsd card in the external slot with two partitions, one of those I use as the primary swap partition.