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soli =NDSolve[{pde, ic, bcs, WhenEvent[{p1[z, t] == 25}, {Print[z]}]} /. 
params, {p1[z, t], p2[z, t]}, {z, 0, 5}, {t, 0, 100}, Method -> {"MethodOfLines", "SpatialDiscretization" -> {"TensorProductGrid", "MinPoints" -> 750}}];

That gave me all sorts of errors. Not to mention it would just print "z". So my question is how do I get it to print out the distance along z?

ss = Table[Plot[p1[z, t] /. soli, {z, 0, 5}, PlotRange -> All], {t, 0, 10, .25}];

soli =NDSolve[{pde, ic, bcs, WhenEvent[{p1[z, t] == 20}, {Print[z]}]} /. 
params, {p1[z, t], p2[z, t]}, {z, 0, 15}, {t, 0, 100}, Method -> {"MethodOfLines", "SpatialDiscretization" -> {"TensorProductGrid", "MinPoints" -> 750}}];

That gave me all sorts of errors. My question is how do I get it to print out the distance along z that it take for p1[z,t] to decay to 20?

ss = Table[Plot[p1[z, t] /. soli, {z, 0, 15}, PlotRange -> All], {t, 0, 10, .25}];