Depends on what distance you want to see it from.
A black hole that glows as hot as an incandescent filament would have as much mass as a 250km cube of rock and it would last for 10^35 years. It would have a radius of 60nm and emit less than a microwatt.
> a black hole small enough to emit xrays
To reach the low end of xrays, 100 electron volts, we'd need a black hole 100 times smaller. It would still have 10^29 years of lifetime, and would be emitting 2 milliwatts of xrays.
To reach the high end of xrays, 100k electron volts, we'd need a black hole 100k times smaller. It would still have 10^20 years of lifetime, and would be emitting 2 kilowatts of xrays.
To go the other way around, if I calculate a black hole that has "only" 10 billion years of lifetime left, weighing a dainty 190 million tons, it would be emitting 10 gigawatts of gamma rays. At 10 gigawatts of output, it would shrink by 1 ton every 285 years. The speed of light squared is an enormous number.
https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...
