Motion of spiral arms of galaxies

I want to find out in what period the spiral arms of
galaxies similar to ours rotate. I am *not* interested
in how fast the matter rotates, but the appearance of
the spiral arms which is quite different.

I understand that one idea is that the appearance of arms
is caused by pressure waves making new stars light up.
In that case, the questions needed to answer this question a

1. What is the velocity of the pressure waves?

2. What is the typical linear distance between spiral arms
measured in the direction of the pressure wave fronts?

Thanks in advance for any replies.

--
Ray Tomes
http://ray.tomes.biz/
http://www.cyclesresearchinstitute.org/