Well, I can't say how long it would take, because frankly it would depend not only on available resources, but also on how much the builders would be able to cut down on labor. As Burki notes in his answer, any naive approach would require impossible amounts of labor. Which given available population and the fact that most of your labourers would only be available for a short time every year they are not busy producing food, would make the time far too long.
I can how ever give some pointers about how to build it.
First, start with an actual mountain of the correct height and roughly the correct shape. No ancient civilization is going to build something of that height from scratch. Why would they? The pyramids and the great lighthouse are about the maximum people of that level of technology might want to build. Anything more would be pointless waste.
There is no useful distinction between "overwhelmingly huge" and "even more overwhelmingly huge". And this goes especially for height. Some temple areas were ridiculously extensive since scaling up in the horizontal plane is fairly easy. Scaling up height in contrast is a major engineering problem that gets increasingly bothersome and pointless as height is increased.
Second, there must be an actual reason to use the mountain as a core instead of simplifying your logistics by building on level ground close to navigable river or safe harbour.
So make the mountain holy. The top of mountain is where gods once stepped on Earth and the ancestors of the ruling dynasty received their blessing, wisdom, or whatever. In many cases the rulers were considered descendants of the gods, in which case the peak would be the place where the first king stepped down from heavens and gave up his immortality to adopt human guise so he could guide the mortals.
In such case it would make sense to have a shrine on the sacred spot and have the royal family live on a palace built on top of the mountain. And then have a court where they receive people and arrange official functions built on a terrace slightly lower on the mountain so that the king does not need to travel up and down the mountain too often. And once you get started building lower terraces, you'll end up having multiple levels for high officials and nobility, for slightly less important officials and nobility, the servants, the guards and so on. You might need a separate level for clergy and temples for sacred rituals and secular matters. You might need a separate level for every level of officials and nobles and such people have infinite ability to discern differences in status.
After you have done such extensive modification to the mountain and its shape to create all those levels, it becomes only natural to do landscaping and architecture so that the overall shape of the mountain is pleasing. In practice, if the mountain roughly resembled a pyramid to begin with, it would be sculpted and filled with masonry until it actually is shaped like a huge pyramid. The process would be gradual one taking multitude of generations.
This is the only way I can think of to get the mega-pyramid with ancient technology and logistics. And obviously since there is no continuous, planned construction, just a constant evolution of form, there is no way to estimate the time needed. In some ways you could consider it a pyramid created by the gods to begin with. That is probably the actual reason it was sacred.