How often do I need to clean my rubber?

doesn't water reck the bat. Also, is it ok to touch the playing surfae of the bat and my dad said that if you play softly or just for fun in a short period of time, you don't need to clean it but if you play hard or you play lightly multiple times, you need to clean it. He belives that cleaning to often leads to reckin the bat. But does it really?

how long does an average rubber last ?

I thought water makes your bat loose grip

AWESOME

I bought an already assembled paddle online and if its rubbers are over what should i do then? Is my paddle useless Alois sir??

I have recently heard that the best is to use your sweat instead of water, and that the rubber would keep the grip for twice as long doing like that as a regular practice. What are your knowledge or thoughts on this? 

And how is it with pimpled rubbers?
DO they need more/less cleaning?
Many players told me that longpips do not need cleaning at all because they do not create spin so there is no need to maintain grip.

Awaiting Alois' answer, I give my experience. I have long-pips on one side. I clean the backside after every training session and the long-pimples side occasionally, carefully, to remove dust, all this with a spongy damp cloth. I seldom use any cleaning product or soap as this gives the rubber a shorter life (according to the shop-assistants/specialists). Please note that there are (just as for backside rubbers), two kinds of pips-out rubbers, one type with friction (not completely blank pips surface) and one type with low friction (each one of the pips ends with a completely blank/glossy/shiny surface) - in both cases you want these pips to be clean to do their job. Long-pips with friction is more for chopping in my view, while long-pips with low friction (and with a pips density more close to 10 (lowest possible/allowed) than to 30 (highest possible/allowed)) is more useful for spin-reversal and deceptive play (and you can chop as well, actually even easier and faster and better, imho). 

But the frictionless rubber has no grip to be taken out,then why it needs to be clean?
I use TSP Curl p1-R.
I have found it to be very adaptable longpips rubber allowing controlled spinny chopping just as well as some corrupted form of even looping.

@D K

I now use SpinLord Dornenglanz. I don't think it is important to clean these longpips. I just wipe them off with the damp cloth now and then (every third week maybe) to remove any dirt, to make sure to keep the low friction playing characteristics the same (if it all matters - can't say I've noticed any difference, cleaning or not). 

I also use the loop stroke with this rubber, works on backspin of course but also on no-spin balls. Fantastic attack shot against backspin, always a winner with good placement and if done faster than the time needed for the opponent to compute the spin vector algorithms ;-)