How long should this last ?

It needs to continue as long as racism exists. Those Milwall fans were very racist and disrespectful.
My long term partner Is West Indian, we like football, we have been to the matches. On one occasion at a away match, which I had driven 150 miles too, a fan called out get the ball you lazy N..... during the match, no one challenged him and when they saw my partner I caught them sniggering. I said something to him and the half wits who laughed with him.
So yes let's carry on with it, and call out this racism. BLM are not saying white lives do not matter they always have, what they are saying is Black lives matter.
These footballers are not show boating, what a ridiculous thing to say, they are highlighting something that really matters.

rosecarmel my post was a riposte to Gagajo's comment The fact is, white lives have ALWAYS mattered*

That comment is not true and I listed white groups that have been treated with utter disregard contempt and cruelty for long periods. The extent of my comment went no further than that.

I am absolutely with the movement to make Black Lives Matter. I just think these gestures become mechanical and meaningless if continued too long but are remembered as special when, like in the UK, we had a limited summer campaign of shouting, clapping and banging sucepans. We will always remember it as special because it had an end point and did not outlive its welcome and gradually dwindle away.