How do I calculate the average runtime of a function?

You can put the code snippet you are measuring to execute within a loop:

import time

REPETICOES = 10
tempos = []
for vezes in range(0, REPETICOES):
    inicio = time.time()
    # ... o código a medir entra aqui ... #
    fim = time.time()
    tempos.append(fim - inicio)

media = sum(tempos) / REPETICOES

Or use the timeit module:

import timeit

REPETICOES = 10

# o seu código fica dentro de uma string mesmo
meu_codigo = """
j = 0
for i in range(0,999):
    j += i * j
"""

tempo = timeit.timeit(stmt=meu_codigo, number=REPETICOES)
print(tempo / REPETICOES)

But you may notice that in both, either "manually" or through the library, the implementations are quite similar.

An interesting way that Python allows you to do is to create a decorator who manages this for you:

def time_statistics(func):
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()

        if not hasattr(wrapper, 'times'):
            wrapper.times = []

        wrapper.times.append(end - start)
        return result

    return wrapper

So, in the function you want to analyze, just do:

@time_statistics
def foo(a, b):
    return a + b

Make time trial calls:

for _ in range(100):
    foo(1, 2)

Then review the results:

print('Tempo mínimo [s]: ', min(foo.times))
print('Tempo máximo [s]: ', max(foo.times))
print('Média de tempo [s]: ', sum(foo.times) / len(foo.times))

See working at Repl.it .

Tip: I used a simple list to manage times, but you can easily abstract this into a more complex structure that already manages all the logic you need, whether you are calculating minimums and maximums, averaging , standard deviation, variance, or generating the graph.