Upfront payment need not necessarily be 'advance' for paying GST. One need to check contract-terms to know its nature.
If contract for lease of 15 years obligates lessee to pay entire consideration upfront, such payment so done will not be 'advance' from GST point of view. And if so, there is no question of proportionate disclosure of rent under GST over 15 years.
And due to reasoning given in my earlier posts, GST on entire consideration (i.e. rent for 15 years) will be payable immediately upon receipt of the consideration after raising tax-invoice of Rs. 10 Lac (i.e. for entire rent for 15 years) plus GST (& disclosure such rent only one-time under GST).
However, contract-terms are as such that said 10 Lac is either 'advance' or 'deposit', then, tax-treatment / disclosure will be different under GST (& same is different for 'advance' & 'deposit').
P.S. As I do not deal with income tax, I cannot comment upon treatment to be given under income tax.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.