If its a solid wood door, proper made, panels etc -
It really depends on the haunches on the lower rail tenon. Theres a built in trimming bit but go over that and the mortice will have no side to it and the tenon will be left hanging in the breeze.
Likely trimming only 5mm max
If its a cheap hollow sounding lightweight thing which is basically a thin wooden frame, of usually pine, one stile usually has a block fitted internally on to it to accommodate a handle/fittings.with a sheet of thin ply glued and nailed to each side.
Usually this framing is quite narrow, and supports the facing panels, and from memory isny any more than 1 1/2", maybe less if it was cheap. the internal is usualy filled with a honeycomb of cardboard.
I wouldnt go anymore that 1/2" off the bottom of it.
Solid fire door or door made of long hardwoood strips glued together -
Cut away- Way-Hey.
These doors can be cut to any size easily, or into 2 or more smaller doors, or panels or anything as its a solid block of possibilities. Makes an excellent workbench surface.
Had a job making and installing a very large front door, plus surround panels, all very nice all in white Oak,bevelled glass the works, really pricy.
Anyway - Fitted, and as its a full renovation of their entry hallway we have floorboards to joists measurements ,made and installed at that.
Bit of teeth sucking, chiselling and gentle persuasion and its in square and in wind, fitted, bit of clean up and we're done.
Then customer tells us theres a new oak floor going in and its not the 10mm laminate we've been working to but now its a full 25mm in solid.
Needless to say there were words, and it meant cutting a fair bit off the door, so it could swing clear, but well beyond the haunched tenons. leaving the bottom rail @ "x60mmx 250mm unsupported at the bottom.
Only solution was to router in a deep slot crossing the mortise, tenon and uncut section of the stile, with a piece of oak glued and screwed into place.
Hope my experience helps you.