You don't want to spend too much time, but you have to spend some amount of time and effort to validate it. I think it scales with the ambition of your idea, but something like 3 months of full time work on well-executed, but non-groundbreaking service sounds pretty reasonable.
The problem with landing pages, waitlists, etc is that they'll give you very little - until people are using your product everything they say is suspect. They need to be using your product, be vested in it, care about it, have opinions about it, etc. Landing pages will only get you a "huh, that's interesting, I might try it out"
To validate it, find several dozen people that will use and love it and give you real feedback. This is a highly manual process and it should be.
The problem with landing pages, waitlists, etc is that they'll give you very little - until people are using your product everything they say is suspect. They need to be using your product, be vested in it, care about it, have opinions about it, etc. Landing pages will only get you a "huh, that's interesting, I might try it out"
To validate it, find several dozen people that will use and love it and give you real feedback. This is a highly manual process and it should be.