Oops, just seen that we're talking about a 151 not 181 in which case cruise is going to about 90kts and so range is nearer 350 nm
I think Johnm makes an important point. Every PA28 is different. Which engine? Which model? Warrior or Hershey Bar wing? New or clapped-out engine? And of course, what power setting? Leaned or not? Head/tail wind? Take-off weight? Which prop, and in what condition?
As a result, there really is no general answer to range, but you can get an approximate answer to duration. The one I fly (a 180) does 38 litres/hr at 75% power (2560 rpm) giving a theoretical endurance of 4 hours 45 minutes, and 32 l/hr at 60% (2350 rpm) giving an endurance of 5 hours 40 minutes. Personally I would never assume more than 5 hours, even though I were flying at around 60-65% power, and I always allow a reserve of an hour on a long trip.
If I'm taking passengers I find the main limitation is that they get bored / want a coffee / want the loo every two hours or so - 3 hours max - especially if it's bumpy.