'incalculable time' is correct and usable in written English.
You can use it to refer to an extremely long or indefinite period of time. For example, "He spent incalculable time trying to fix the broken machine."
When Chevy was blown from the Stryker hatch, he took flight for an incalculable measure of time before landing on the slat armor of his vehicle.
Lately I've spent an incalculable amount of time thinking about the experience of being a twenty-something.
Brant took the idea of spatial music to a new level of power and grandeur, and his impact on the music of his time is incalculable.
He said that the agency's work is of incalculable importance "at a time when disarmament efforts appear deadlocked, when there is a danger that nuclear arms will spread both to states and to terrorist groups, and when nuclear power again appears to be playing an increasingly significant role".
Destruction "culturally speaking an incalculable loss" - Nepali Times editor Kunda Dixit.
The amount of money invested by various companies on research and development over this long time is almost incalculable.
Aaron Parks ARBORESCENCE "Arborescence" (ECM) is the first solo piano album from Aaron Parks, who has proved, time and again, his incalculable gifts as an ensemble player.
Farage has stood and lost in almost as many parliamentary contests as the Monster Raving Loony party, but the consequences of him losing at this time would have been incalculable.
But we live in a time when imponderable and incalculable increases in data are fueling the ultimate "Revenge of the Nerds": A crying need for super-geeks with the gift of knowing how to make sense of all this information.
"With a conviction that seemed to border on political suicide, the authorities bet all or nothing, and this time the returns will have incalculable reach," Max Colodro Riesenberg, a professor at the University Adolfo Ibáñez, wrote in a newspaper column this week.
