how much time would it take to clean fuel tank?

Well,I don't know about that or what the mechanic charged you to do that,but it don't really sound right. you can order a brand new fuel tank for less than $125.00 and that's including shipping charge (about $10.00) for 2-3 business day delivery from rockauto.com! local parts store is around $235.00 for same exact part! copy and paste link below.... https://www.rockauto.com/en/catalog/toyota, 2001,echo,1.5l+l4,1375220,fuel+&+air,fuel+tank,6268

It most likely did indeed take that long to remove the tank - remove and disassembled fuel pump module - clean tank plus re-assembly and install..