The Halachos of these things are very complicated. It’s really best to ask a shailah. Some things an individual may be makpid on, and then there are things most people are makpid on.
For example if a person has always told you, I don’t want a shidduch if the person has a mental illness in the family. You may have more of an obligation to tell them initially than someone else who should perhaps be told later on.
If you’re lucky enough to have time to get back to them and didn’t yet reveal this information – then go ask a shailah.