Radiator Question

How long would a head gasket have to be blown before oil showed up in the radiator\ coolant?

It depends where the gasket is blown. It may NEVER show up in the radiator.

It depends where the gasket is blown. It may NEVER show up in the radiator.

Well let's say that it did, would it show up in a day? a month?

Normally I've found anti freeze in the oil look for foaming.

Well let's say that it did, would it show up in a day? a month?
If the HG blew in that way, the coolant would start leaking into the oil virtually immediately, as soon as the engine warmed up and the cooling system developed some pressure.
Look at the engine oil dipstick. As soon as the oil starts to look foggy and opaque, like a chocolate milkshake, the coolant has made its way to the oil.

It depends where the gasket is blown. It may NEVER show up in the radiator.

If a head gasket leaks from a combustion chamber to a coolant passage, the result would be compression entering the cooling system and most likely a hose, the heater core, or the radiator would burst. The other possibility is coolant being sucked into the combustion chamber on the intake stroke, in which case, coolant would be expelled out the exhaust. Oil passages in the head gasket are not under pressure so they can't leak into the coolant passages.