http://bikecalculator.com/index.html
By plugging in some basic info about yourself and the climb it comes with an estimate of the time to climb. I have found it to be remarkably accurate.
By way of illustration the table below shows some numbers for a variety of different riders on the principal climbs used on the Marmotte. (The "pro" is David Lopez, data from the 2013 TDF stage where they climbed the Alpe twice, shows the difference between "tempo" and taking it "easy")
The difference between actual and calculated is small. Even where they are "wrong", on the Glandon, it illustrates an interesting point. This climb as a vicious little V valley midway up that will slows you up. It is also the first climb in the event so it gets congested and time is lost working your way through the field. Those going faster and/or starting at the front suffer less from this.
Rider | Kg | Climb | Distance | Gradient | Power | Act Time | Calc Time | Difference |
Pro | 67 | Alpe D'Huez | 13 | 8 | 360 | 45 | 43 | 2 |
Pro | 67 | Alpe D'Huez | 13 | 8 | 288 | 55 | 52 | 3 |
RiderA | 74 | Alpe D'Huez | 13 | 8 | 313 | 54 | 52 | 2 |
RiderB | 71 | Alpe D'Huez | 13 | 8 | 222 | 70 | 69 | 1 |
RiderC | 70 | Alpe D'Huez | 13 | 8 | 247 | 64 | 65 | -1 |
RiderD | 70 | Alpe D'Huez | 13 | 8 | 263 | 58 | 59 | -1 |
RiderA | 74 | Galibier | 18 | 6.6 | 306 | 66 | 65 | 1 |
RiderB | 71 | Galibier | 18 | 6.6 | 224 | 84 | 81 | 3 |
RiderC | 70 | Galibier | 18 | 6.6 | 223 | 82 | 81 | 1 |
RiderD | 70 | Galibier | 18 | 6.6 | 241 | 74 | 75 | -1 |
RiderA | 74 | Glandon | 23 | 4.8 | 331 | 64 | 62 | 2 |
RiderB | 71 | Glandon | 23 | 4.8 | 254 | 83 | 74 | 9 |
RiderC | 70 | Glandon | 22 | 4.8 | 274 | 74 | 69 | 5 |
RiderD | 70 | Glandon | 23 | 4.8 | 245 | 81 | 75 | 6 |
RiderA | 74 | Telegraph | 12 | 6.9 | 335 | 40 | 41 | -1 |
RiderB | 71 | Telegraph | 12 | 6.9 | 250 | 50 | 51 | -1 |
RiderC | 70 | Telegraph | 12 | 6.9 | 259 | 50 | 49 | 1 |
RiderD | 70 | Telegraph | 12 | 6.9 | 241 | 48 | 52 | -4 |