To test or not to test that is the question

Gemma • I have 3yo twins from a prev relationship, hoping to have baby #3 with my wonderful fiancé. Been trying for 16 month xxx
Oh em gee!!! I'm spending a fortune on home testing kits because I just can't help myself but to test, then I get let down with a negative result so my question is when is the ideal but not too long time to test?? Xxx