How do you handle this fine line??

[wiper]

every night, my girlfriend comes home, and immediately starts talking (complaining) about her job. it drives me up a wall, because i have long days too, yet choose not to bother her with the details. however, i do like to be left alone for an hour to decompress..

anyway, i've learned to nod my head, and occassionally chuckle at her boring stories about everyone at her school.

my question, and problem, is this...

how do i act interested enough to get her to not bitch that i'm not paying attention, yet not enough for her to think that i really AM interested, and for her to expand her stories to an hour or more??

it's a fine line, and i'm interested to know how other boyfriends/husbands deal with this...

case in point: about 5 minutes into her gripes, i chuckled (half-heartedly) at something about her boss. i meant it to show her that i was paying attention (i wasn't, and actually wasn't even sure that it was the right time to laugh), but she took it to mean that i wanted more...'haha..yeah, you know, lemme tell you something ELSE!'..

ugh.

anyhow, any tips? i know it isn't the biggest gripe in the world, but at 5pm, after being home 10 minutes and enduring 4 classes of adhd/autistic/asperger's students all day long, the last thing i want to do is hear another teacher gripe about her long day...