I used to do the 1 hour per month in-services but found that a lot of my guards weren't coming. Currently I am doing a 2 hour in service once per session (every 7-8 weeks). I give the guards 2 options for days/times and they have to attend one of them. I usually do a Saturday or Sunday afternoon and a weekday evening. It's a little more work on my part, but it makes sure I get everyone there.
When I was assistant manager at a pool, we had a mandatory monthly meeting on Sunday mornings before the pool opened where the guards got their monthly reviews and updates from the management staff. We then had the in-service following those meetings since the lifeguards were already required to be there.