If you are expecting a number, sorry can't give one.
However these are the variables you will have to consider
- Bike - some bikes shimmy at high speed. This can be caused by minor unbalancing like a yellow spoke reflector, or a small variation in rim weight and balance.
- Wind speed and direction - side gusts have a greater impact at speed.
- Traffic - Cars can get in your way, plus they act a bit random when cyclists start doing unusual things
- Road - a descent often goes down a hill, and tends to wind in and out of valleys and bluffs. Your line of sight can be quite limited by the hillside or vegetation. So a corner may cry out for shortcutting, but you run the risk of going head-first into an uphilling vehicle or other rider.
- Weather - road temp and dampness will affect tyre grip. Heat will cap your ability to cool down by sweating
- Reaction time - as you move faster you cover more ground. Means you need to anticipate better and to predict further ahead.
- Wind - if you go fast enough you get cold ears and eyes, nose, fingers and toes. Also, the passing wind can set up a howl or whistle as it passes your helmet straps. I find this may be one cause of my gradual hearing loss.
Ultimately the limiting factor is likely to be your Confidence, and the point at which you reach for the brake levers.
To calculate your likely downhill speed, you need to know your CdA, your input power, and so on. A website like http://bikecalculator.com/ helps give a guesstimate.
One example, assuming you're putting in 50 watts of power (and that you're not spinning out the gears) you weigh 90 kilos, your bike is 10 kilos, the grade is 5% down, there is no wind relative to the ground, its a 10 km long segment with an air temperature of 25 degrees C and its 1000 metres above sea level (not sure if thats start or finish elevation sorry) and that you're in the drops the whole way, you'll be doing 62.7 km/h and do the whole thing in ~9.6 minutes.
If you don't pedal at 50W you'll do 60.9 km/h and do the whole distance in ~9.86 minutes, so 17 seconds slower.
Of course these numbers are completely calculated, so you play about with the inputs. EG a 250 kilo rider on a 50 kilo MTB bike would do 81 km/h on coasting ?
On most downhills you spin out your gears quickly. http://www.sheldonbrown.com/gear-calc.html is another good calulator for this.
It shows that a 52 tooth chainring with an 11 tooth rear would give you 71km/h at 120 RPM, assuming normal road wheels. Thats really fast pedalling and feels crazy. At 90 RPM that same gear combo would be 54km/h. Hence why I used 0 Watts in the previous calculator, meaning coasting.
Lastly - do ride with something like strava or some other recording tool to measure how well you did. This also gives you fairly accurate proof for next time you do the ride to compare.
Doubtless there will be strava segments defined so you can compare your output with other riders too.
And ultimately, your time to ride UP the hill is just as interesting :)