How to find the number of values the equation outputs before repeating the same outputs again?

This script is based on implementing Shor's quantum factorization. For the equation 'fncT' whicn is alternative version to mod(), it outputs 4 different values before repeating again, given the parameters 'T = 7' and 'M = 15'. Is it possible to get this number 4 using the fft function and assign it to a variable instead of manually assigning it?

script:

T = 7; % T^P mod(M)

M = 15;

P1 = 127; % 128

P2 = 255; % 258

P3 = 511; % 512

P4 = 1023; % 1024 bits

t = 1:1:sym(P1); % Signal duration/length

fncT = (T.^t) - floor(T.^t/M)*M;

%plot(t,fncT)

%stem(fncT)

A = fftshift(double(fncT));

A = fft(A);

A = fftshift(A);

A_mag = abs(A.*A);

stem(A_mag)

%plot(A_mag);

% GCD

eqn = T^(4/2) + 1; % 4 = no. of time it produces different values before repeating again

eqn1 = T^(4/2) - 1;

[G1] = gcd(eqn,M);

[G2] = gcd(eqn1,M);

disp(['The factors of 15 are ',num2str(G1),' and ', num2str(G2), '.'])

[pks1,pks2,pks3] = findpeaks(A_mag);

x = pks2(1); % Assigning the peaks with a variable

y = pks2(2);

z = pks2(3);