I replaced the batteries in my flashing lights last night. What a huge difference it made! I thought they looked OK, but decided to replace them anyway to see if I noticed a difference in brightness. I think the brightness dims so gradually over the course of a few weeks that I don't notice how dim they become. I've tried to think of a way to prevent this and install batteries before it becomes an issue. I'm going to write the date I replaced the batteries on a piece of tape and stick it on one of the lights. That will give me an idea how long the batteries have been in there.
If anyone has other ideas for remembering when to change batteries in the flashing bike lights, let me know.