hard sci fi - How long could Miller's Planet remain in orbit

In Christopher Nolan's movie Interstellar, Miller's Planet is in orbit around a supermassive black hole affectionately (??) called Gargantua.

Miller's Planet orbiting Gargantua:

It is orbiting "at the cusp" of the black hole, in an orbit I presume to be very close to the "critical orbit" (which is the closest stable orbit to the black hole - inside this radius not even light can orbit stably). Kip Thorne specially designed Gargantua with a high spin so that Miller's Planet's orbit would be "stable".

But how stable exactly?

How much time would pass on the surface of the planet (don't forget relativistic time dilation) before Miller's Planet slipped inside this "critical orbit" and began is spiral into Gargantua?

Answer

Orbit of this planet wouldn't be stable because it would have to cross accretion disk or paths of particles falling from it into hole - it would cause significant drag and cause deorbitation - it would fall into the black hole.

Another thing is that this photo seems inaccurate: being near photon sphere, planet would be much closer - in similar distance as inner edge of disk, probably too small to be seen from such perspective.

Even black hole itself doesn't look right for the scenario the movie creators cherry picked to have extreme time dilation on the planet - very high rotation rate of the black hole was needed and light coming from the disk would be distorted differently from what they used in the movie - a much lower rotation rate version of simulation of the black hole.