Ammar,
You can print the result. Also, As Marco points out, Since Plot extrapolates beyond the end of your integration (Evaluate sol1 to see this) , you can save the end value and use it in the plot.
I would do:
ode1 = {x'[t] == -1, x[0] == 1, y'[t] + y[t] x[t]^2 == 100,
y[0] == 10,
WhenEvent[x[t] == 0, "StopIntegration";
Print["EndTime: ", stoptime = t]]};
sol1 = NDSolve[ode1, {x, y}, {t, 0, 10}]; Plot[{x[t] /. sol1}, {t, 0,
stoptime}]
Regards,
Neil