How long does the business person asking for it think it will take? Double that.
Is there a model system that it is being compared to? Double your estimate again. Double your estimate again if it’s being compared to more than one system.
Is it the design going to undergo audits for standards or compliance? Add 50% for each.
Does Agile mean “work before requirements are figured out in a process that’s really Waterfall”? Add all the effort up until your last new specification to the end of the project timeline.
Is “concept” or “pilot” being used in the place of “live production product that will be expected to scale and configure from day 1”? Double your estimate again.
Congratulations! You now have a very conservative estimate for how much effort things will take.