In reply to Adarsh Santhosh:
The strong qualifier applies to a sequence that is a property.
That makes the property strong.
below works
module m;
bit clk, done, a;
initial
ap_1: assert property(@(posedge clk)
done[->1] |=> (!a[->1])
#=# s_always[1:100000] !a);
endmodule