There's a bunch of questions that commenters have already raised but I'm going to give it my best shot. I'm assuming that the metal object is like the 'sun' for this planet, so this planet orbits it. No idea if this is feasible, but I'm going to assume it is (i.e this metal thing has enough mass to attract this planet of yours).
First of all: planets don't exactly collapse. The entire point of the formation of a planet is that a bunch of heavier-than-air matter collides and combines into a denser mass. This can be said of all rocky bodies in our solar system.
But you are describing a hollow planet. Not sure if this is scientifically possible but assuming massive hollow spheres work the same way as smaller ones, we know that the inside of this sphere will have a net gravity of 0 (assuming all mass of hollow sphere is distributed evenly across the surface). Outside the sphere, this planet's gravity pulls towards its center. How would this planet collapse? With some big moves, that's how:
- Large impacts: an impact big enough to put a significant hole in this planet, making this hollow sphere a hollow sphere with a whole in it. This planet is in fact planet sized, so this won't necessarily collapse the planet unless the whole was big enough, but it's bound to cause issues. This would violate the net-zero gravity on the inside of the planet, and little by little the planet would likely disintegrate and turn into a ball of mass (like planet Earth) or turn into a ring around that metal object that this planet is apparently orbiting. Fun fact: the asteroid belt is thought of as a failed planet. Maybe this is what went down...? Probably not but cool thought. This planet was originally, however, created well (all weight distribution across surface was uniform enough). Keep in mind that this uniformity doesn't have to be perfect (so small hills and stuff would still be ok on the surface) but it can't be wildly inconsistent.
- Uneven weight distribution: If one part of the sphere is significantly denser than another, we won't have the uniform net-zero gravity thing happening in the hollow part of this sphere. Again, the problem here is not with the spherical nature of this planet, but with inconsistencies in its mass-distribution. But upon creation this planet must have been stable - this weight redistribution must have taken place after the planet was created.
- Terrible construction: If the internal surface of the planet is made of denser, but more brittle material, and is inconsistent-enough distribution of mass across the inner-surface of this planet (violating the net-zero gravity inside the sphere), we could have problems. Let's say this material is large chunks of iron - eventually these chunks of iron will find their way into the center of the hollow planet and take other bits along with it, possibly leading to a slow but exponentially growing rate of disintegration of the planet.
I don't have much math to back this up but if I find something I'll edit my answer. I hope this helps.
EDIT1: I just realized I didn't technically answer the question. My answer is that mass doesn't matter, mass distribution matters and anything that messes with mass distribution matters. That causes destruction/collapse. So, if this planet had the mass of Jupiter, was hollow and only 1 km thick at all parts of its surface (mass perfectly evenly distributed and perfectly constructed so problem 3 doesn't become relevant), with no holes or anything, there's a good chance it would be perfectly stable with no chance of disintegration.
EDIT2: Use proper materials when building it. The only way any such planet could realistically be built and not fall apart is if you used really strong materials (stronger than diamonds and carbon nanotubes, maybe make something up) to hold it together and maintain its structure.