With Grant leading the assault force, a rarity of competence among the generals on both sides: 100% chance of falling on the 16th or perhaps 17th.
With 16,000 men to defend a roughly 2 mile defensive perimeter, the Confederates were fielding 8000 men per mile. Despite numbers seemingly prophetic of World War 1 the fort succumbed on the second day of assault.
The truth is that Fort Donelson was woefully unprepared for an overland assault; doomed to fall quickly to any competent, determined, and aggressive assault. Think Singapore in February 1942. There was no fallback position from the outer perimeter except retreat into the fort proper combined with surrender (or as attempted unsuccessfully, breakout) by 80% of the defensive force; but even that buys, at best, only a few extra days.
General Simon Bolivar Buckner, however, argued that they were in a desperate position that was getting worse with the arrival of Union reinforcements. At their final council of war in the Dover Hotel at 1:30 a.m. on February 16, Buckner stated that if C.F. Smith attacked again, he could only hold for thirty minutes, and he estimated that the cost of defending the fort would be as high as a seventy-five percent casualty rate. Buckner's position finally carried the meeting. Any large-scale escape would be difficult. Most of the river transports were currently transporting wounded men to Nashville and would not return in time to evacuate the command.
A properly prepared fort holds out for months instead of hours, as Mantua in 1796 (7 months), Genoa in 1800 (2 months), Mafeking in 1899-1900 (7 months), or Tobruk in 1941 (8 months). Note that the number of defenders above for all but Mafeking is not far off the 16,000 at Donelson: Mantua 16,000; Genoa 11,000; Tobruk 27,000
All meaningful land-facing defense of Donelson was encompassed in the assumption that Fort Henry wouldn't fall; with Henry itself having little in the way of land-facing defense. Although the Union wasn't big on that sort of thing: any Union commander at Fort Henry who didn't promptly capture Fort Donelson would have deserved court martial for cowardice in the face of the enemy.