How big has to be the invest and how long will take to start some profit out of it ?
No one can really tell, it depends on many variables and some of them are unpredictable: obviously Bitcoin price cannot be predicted in the future, same goes for the evolution of difficulty, ASIC technology, etc.
My opinion is that you are more likely never to see any profits out of mining unless you are lucky enough to live in a place with the cheapest electricity on the planet, or willing to move, or at least travel there and setup a local maintenance team (who needs to be highly trusted obviously).
If that's not possible at the moment , there's people minning for fun ?
Does people that mine lose $ ?
Some miners do (mine for fun and lose $), but I assume that institutional miners are just at the threshold of profitability: they need to keep mining to pay for their fixed expenses (datacenter rents, staff, etc.) and electricity, but they are no longer making any profits. Some institutional miners have already shut down their miners (e.g. CEX.io cloud mining), and others will probably follow suit, re-adjusting the difficulty lower, thereby allowing the remaining miners to stay above the profitability threshold.
It's a kind of equilibrium.
One thing is for sure: the "buy an ASIC and get rich quick" days are over, actually it only really worked for Avalon batch 1/2 pre-orders Q2/Q3 2013 + some KnC miners Q3/Q4 2013, mainly the Jupiter or Saturn.
