how reliable are IMUs for a long usage (6 hours) rotation (only) tracking?
MEMS gyros, which are rate gyros, when integrated to produce orientation angles, generally have high rates of drift that are due to offset errors and noise. You can find these values in the datasheets.

This can be mitigated by finding another source of data that provides an absolute reference. For example, you can use the accelerometers to measure the gravity vector, which stablizes the pitch and roll measurements from the gyros. And if you have a magnetometer available, you can use it to stabilize the yaw measurement.