I re-read the CO thread in the best of category and came across this link to some testing....http://www.ncbi.nlm.nih.gov/pubmed/21536403
in case of copyright issues i did not copy and paste the info here.
The conclusion of their controlled testing is that the co does not stratify and will mix readily with the air in the room so co monitor height is not critical.
common sense tells me the speed the co monitor alarms depends on how fast the air in the house mixes - so how fast is that...pretty quick in my basement sample test size of 1.....within 2 - 4 minutes!
my real world experience has a monitor plugged in 30" from floor level in the basement (ceiling height of ~7') about 8' from the stove. i had reconnected everything on the stoker this past saturday and fired it up to make sure it was all OK and i would be ready when it was time to light up for the season.
on the monitor prior to lighting i checked max level and it was 0, i reset it anyway. i saw a reading of 21 on it within 2 minutes of lighting the cowboy charcoal. i wasn't surprised since i had the door open with the combustion fan on to light the charcoal with a propane torch. and after closing the door i could see some of the sparks from the cowboy charcoal forced out around the air wash space on the glass while the manometer read .01 draft, exhaust probe in the flue was less than 100* at that point. as the cowboy coal caught after about 2 more minutes and the firebox warmed up draft increased to ~.03, exhaust probe ~110* and no more sparks through the air wash, monitor readout indicated 0, max reading was 21 which i then zeroed.
I turned off the combustion fan and opened the door and added a few shovels of rice coal to the grates over the cowboy coal. closed the door and turned the combustion fan back on. took about a minute, but while the door was open i could smell the cowboy coal although i didn't see smoke. i looked at monitor and the display reading still showed 0. i watched the grates to make sure rice coal was starting to burn good, waited about 3 more minutes before powering the controller so the feed motor would start. the reading on the monitor was still 0 but i hit the max reading button and it read 18. i am guessing the 18 was from when the door was open while adding rice coal but i didn't see that number show up on the display like i did the earlier 21.
i reset it back to 0, didn't open the door any more, the stoker got up to operating temp, draft rose to ~.05 (i don't have the baro in this time of year, i still have the T capped), temp probe on the flue went up to ~280*. the display and max reading stayed at 0 throughout the evening and the next morning.