Solo founder; it tends to change rapidly depending on the stage of the product.

Probably 90% of the time writing code currently. Once the hardware product is released publicly it drops to 10% (maintenance) and then ramps up when new significant features are added.

Broadly speaking..

Stage 1: design specs 0% code

Stage 2: prototyping 50% code

Stage 3: the hard work 90% code

Stage 4: selling and maintenance 10%+