what would happen in a rag was caught in the turbo

Well if it made it through the turbo it would end up in the throttle body. I suppose it would eventually by chewed up and burned to a crisp before sent out the tailpipe as ash and smoke. Can't say it wouldn't toast the turbo in the process though.

would it cause any symptoms and how soon.

What exactly happened, was the rag placed in the turbo or exhaust tube? If it's just stuck in the tube and not sucked through, I imagine it would just cause restricted air flow, which would hamper performance and fuel economy, but it should burn up at least. Otherwise it may damage the turbo, throttle body, or exhaust valves. I think it would be worth it to open it up and remove it.

If the rag git sucked in through the filter, then the problem may be with the vanes on the turbo, if one got bent or broken, as well as clogging the intercooler. I think in that case the effect would be felt pretty quick.