We need to change this a little bit - we would say something before 'spare'. We could say, If I have some spare time, or else, If I have the time (or, If I have time). Those would be the best options. Best wishes Steve

It's fine, there are tons of shortened colloquialisms that can be used in place of it but overall it's correct. This movie's great, I'll watch it again when I can -once I have the time -in my spare time