Depends on the timing of arrival. If the aircraft door opens before the immigration rush, you can get through into to baggage claim in minutes. Baggage is almost always fast in Japan.
The bus timetable uses the worst case transfer times along a standard route (eg, the expressway through Tokyo and Chiba). The bus company monitors traffic conditions and reroutes buses to avoid the worst traffic. Depending on the traffic, the bus can reroute through the Aqualine and avoid a lot of the traffic through Chiba. The Tokyo/Yokohama end of the Aqualine is right next to HND.
The bus between airports itself can take anywhere from 45 to 75 mins, depending on the time of day, which route is used and the traffic conditions.
The only time travel from NRT to anywhere in Tokyo has taken me more then an hour has been when I use the normal JR train (not the Narita Express) or used the N'Ex to somewhere past Tokyo Station.
It is doable to make the transfer between airports, including immigration, baggage collection and transport confirmation/boarding in 80 mins if everything works out timing wise with no delays. You need no line at immigration, bags coming out as you get into baggage claim and the bus ready to go just as you get on.