Hmmm.. this is a tricky one.
I've been doing some miles in mine in the last fortnight, and as the washer bottle emptied, the light definitely came on, as it totally surprised me.
At that point, the lights stop washing anymore and you only get windscreen, but then I'm pretty sure the washer-empty light only stays lit actually around that time..
for example.. next day, washer bottle is almost empty, no light. when it actually emptied, again no light.
once id filled it up, it took heeeaapppps of water to get the lights spraying again too. i was quite surprised how dim the xenons get when they're covered in poo.