Find maximum length of array where every entries are the same
You should use better function names ALG1 and ALG2 don't really say what they're performing.
You should keep the creation of A, n and result at the bottom of your code.
You should keep your 'main' code in a main guard block:

if __name__ == '__main__':
    A = [3,2,4,2,4,2,4,5,3]
    n= len(A)
    result = ALG1(A,n)
    print(result)

You should follow PEP 8, so that it's easier for others to read your code.
ALG2 can be simplified by slicing the array from i to j, A[i:j], and then checking if the set has a size of one.

def ALG2(A, i, j):
    return len(set(a[i:j])) == 1

Your entire code can be drastically simplified by using itertools.groupby.

import itertools


def ALG1(array):
    sizes = (
        sum(1 for _ in g)
        for _, g in itertools.groupby(array)
    )
    return max(sizes, default=0)