c[l_] := Sqrt[2/3*l/9.81]

schritt[{s0_, v0_}, l_, dt_] :=
 {s0 + v0*dt, v0 + Sin[s0]/c[l]^2*dt}

Zeit[s0_, v0_, l_, dt_] :=
 For[n = 0, Nest[schritt[#, l, dt] &, {s0, v0}, n].{1, 0} < Pi/2, n++,
   Print[n]]