I have an AIS receiver, mounted on my stern rail, about five feet above sea level. This gives me a range of about five miles, a bit more for a large (tall) ship.
Sailing out of Guernsey, the fastest ship around is Condor Liberation, a fast ferry
, usually doing 38 Knots. At that speed I have about twelve minutes notice, which I consider to be plenty of time, especially as I can see the CPA and Time of CPA on both iNavX
, plotted on my chart.
Out of interest, I have occasionally connected it to my masthead aerial, and this can give me signals from the other side of the English Channel
(sixty miles) on a good day, but reliably, about twenty miles. The Channel is very busy, of course, and seeing all that traffic is too much information.
I have Radar
, too, and in bad viz it is either on four or eight miles range, where I can see what is important to me.
Another advantage of having the aerial on the stern rail is that it is an independant aerial, and can easily be switched over to the Maeine VHF, should the other aerial fail (dismasting?).
Enough information is enough, and any more is just distracting.
I hope this is the answer you are looking for.
Many, many boats do not have an AIS transmitter, so Radar is still more important, in my mind, than AIS. You could consider a Radar active transponder, such as the SeeMe as an alternative.