Usually how long does a concert last? say a rock concert?

It depends on the number of opening acts that are playing. Ive found that normally with three openings acts the show starts at 7:30 and is out by 11:00 or 11:30

Usually around two hours... it depends on the concert.

Depending on how many songs are played and the amount of intervals, support bands etc. anywhere from:
1hour - 6 hours.
Then you have festivals, such as Soundwave etc. that can last up to 12 hours.

2 hours

Usually the main band will play for about 90 minutes to 2 hours. Of course there are often 1-3 bands opening.

Most concerts have a main headlining act and two support bands.

The first support band will be a local band or a band who have just broken thourhg and will play a short list of between 30 minutes and an hour.

The second band will be nearly as famous as the headline act and will play at least an hour.

The headline act will play anything from 90 minutes to 4 hours, depending on their stage production and the length of their songs!

for the ones I've been to, 4 - 5 hours depends on the band and type of music i guess. but for rock around 4 - 5 hours.