Ḥijjah here is a synonym for sanah, a year. Two decades help explain why the ruined site is difficult to identify. Tawahhum, however, is not a measure of time but a mental condition: the speaker has not yet moved from a vague impression to certainty.
TermMeaning in context
حِجَّةYear
لَأْيًاWith difficulty
تَوَهُّمUncertain supposition