How long before I see an effect from this?

I used it earlier in the year, and TBH I didn't notice a great deal of
difference.

Used it last weekend again as I noticed some broadleaf weeds coming through
the grass and again there doesn't seem to be a great deal happening. Am I
just being impatient?

The turf was put down last August, and although we did our best to get rid
of all the weeds you never manage to get them all, so we've got some
dandelions and dock coming through the grass.