What should be size of data for SISO system identification with sustem identification toolbox

Greeting!

Can some one please guide that how much data is enough to identify a model in system identification toolbox. Is there any rule of thumb.

I have alot data for a SISO system ; multisine inputs is used with one period 4000 samples and total are 50000 samples. I observed that different size data gives different results for the specific model characteristics.