One needs to differentiate between thermal waves and ridge lift induced by thermals.

Real thermal waves are actually quite rare, at least here in the German flat lands. They require a significant wind-shear in direction and strength above the inversion layer. Cloud-streets lining-up 90° to the actual wind direction below inversion are an indicator of such thermal waves. These are real waves that bring you way above the convection layer. There were reports of altitudes 4000m AGL (cloud base at 2000m).

Climbing in front of a Cu like in front of a ridge happens quite often. All it needs is a wind increase above the condensation level. The lift in most cases is not very strong and brings you a few hundred metres above cloud base level. I had one occasion earlier this year that brought me to the Cu-top, some 800m above cloud base.