A= 2; % amplitude = 2
f = 10; % frequency = 10 Hz (a.k.a. 10 cycles per second)
N = 4; % number of periods = 4 (a.k.a. 4 cycles)
t_max = N/f; % N cycles / f cycles per second = N/f seconds <- time required for N cycles at frequency f
t = linspace(0,t_max,1000);
x = A*sin(2*pi*f*t);
plot(t,x)
You can use t = 0 as the left x-limit, then the right x-limit is t_max = N/f as given above. Set the x-limits in your GUI like:
set(ax,'XLim',[0 t_max])
where ax is your axes.
