how to find out when an EC2 instance was last stopped

You could use the reason variable of stopped instances:

import boto.ec2
conn = boto.ec2.connect_to_region("eu-west-1")
reservations = conn.get_all_instances()
for r in reservations:
    for i in r.instances:
        if i.state == 'stopped':
            print "%s [%s] %s" % (i.id, i.state, i.reason)

Output:

i-11223344 [stopped] User initiated (2013-12-20 13:59:08 GMT)

This works also for terminated instances (as long as they are still displayed).

I think that better practice would be:

import boto.ec2
conn = boto.ec2.connect_to_region("eu-west-1")
reservations = conn.get_all_instances()
instances = []
for reservation in reservations:
    for instance in reservation.instances:
        if "Name" in instance.tags.keys():
            instances.append((instance.tags["Name"],
                              instance.get_console_output().timestamp))

You can also replace the if and get whatever you want, but instance.get_console_output().timestamp is the right way to get instance's stopped timestamp