Kim has a great article on long exposure photography and calculating exposure values. At the end of her article is a quick table on some common values for a 10 stop filter.
While you can note these down and take them on location with you there are many gadget options. One I like is a small free app for your iPhone called LongTime Exposure Calculator By HPR-Solutions. You dial-in the filter’s number of stops, choose the shutter speed indicated by your camera (before attaching the filter) and the app tells you the new shutter speed with the filter attached.
There isn’t a countdown timer, audible signal or the option to use aperture instead of shutter speed. Nor will it operate the iPhone’s camera. I’ve seen these criticisms elsewhere, but to me the app is simple and does its job well; exactly what I need.