I can tell the blade is getting a little dull when
- it 'pulls' across the skin
- i need to run it over a couple times to feel smooth when i didn't need to do that before
- i find myself using more pressure than i used to (thats probably how come it feels like it is 'pulling' the skin)
- i start to get red bumps on my skin after shaving

good quality blades tend to last a while, though. even when the above starts to happen i still usually get a couple more weeks out of the blade (shaving legs and pits every 2 or 3 days)

A refill pack of blades (about 3 blades) used to last me about 9 months...

I have a Braun Epilator too :-)
I love it :-)

I do my pits with it as well. I even did (most) of my bikini with it for a time. I stopped doing my bikini, though, so I'm back to shaving it once it gets passed that 'trimmed' look... I really hate shaving once it gets to that, though, the skin doesn't like it at all and since I just get cheap disposables now it takes a good couple razors...