Max watt bulb for an old "660w 250v" table lamp?
Without any other information, you are left with trial and error. With an incanddescent bulb you need to be concerned about overheating the shade. With an LED bulb the concern would be the temperature of the control circuit in the bulb.

The 660 watt rating of the lamp holder would indicate that should never be a concern, other parts of the lamp will always be the limiting factors.

Unless the shade is ridiculously small, I would think a 60 W incandescent would be fine and 100 W probably ok. For 150 W, you should probably have a larger clearance between the bulb and shade than would be usual for a single-bulb lamp. The safety issue comes down to the possibility of the shade catching fire, but you might also want to avoid discoloration.