NDTimer 4+

A fantastic app to help with long exposure photography. Just been on holiday for a week where I tried this app when using my 10-stop ND filter. Great results every time. What sets this app apart from others that I tried is that it has an in built timer which is invaluable. Not experienced the timing bugs experienced by another reviewer although I have to say that I have only taken about 20 or so phots using this app. When perched on a rock taking long exposure photos of a waterfall you don't want to be moving around too much so the benefit of having everything you need in this one app to work out the exposure is invaluable. Well worth the minimal fee.

There are several of these LE calculator apps around. This one stands out because it is a) virtually free (69p is about a 1st class postage stamp), b) has a timer for shots over 30 secs, and c) allows you to add a 2nd filter such as a polariser or grad before it does the calculation.
For me those three make it the winner.
Have not experienced the timer problem but stopping iPhone going to sleep in Settings seems sensible.

I've tried a couple of these ND timer apps and this has some strengths over others, but also some flaws. On the plus side, good range of shutter speeds, and it calculates to the second. On the bad side, the built in timer is horribly buggy (on screen time stops counting down, but some internal timer does still work), especially if the iPhone goes to sleep on a long timer, which is doubly annoying because if you try to avoid your iPhone sleeping, your quite likely to adjust a setting and restart the timer (please lock the settings when the timer is running), and the stacked filter option is always forgetting the settings.