I have been asked a few times, how much is enough for retirement on a monthly basis. I would usually reply, how much do you spend in a month.
It's not too difficult a sum to work out. But one should always assume all costs involved, including loans and leisure expenses.
I find most retiree aspirants do not attempt to factor in costs of their entertainment and leisure activities. This results in a budget that crimps their current lifestyle. Thus, making for an almost painful retirement.
Go ahead and factor in the holidays, gifts, shopping, movies and gambling jaunts.
You would then realise you need much more than you initially thought.