Thanks so much for all the replies!
Now, please, how much do you think would be a "good" amount for selling today in:
Bitcoins
Peercoins
Litecoins
Dogecoins
Some other coin that could be a good example
the best example is to think about the average exchange volume, on the exchange(s) you will be trading on, or the average across all.
if your amount to sell represents a significant % of the average volume over a given time period, you will definitely affect the market.
example
you want to sell 3x the average hourly volume of LTC on cryptsy.
you will push the price down if you try to sell it all in an hour because you exhaust all the buy orders at the currently traded price. if you spaced it out over a day the effect may be minimal.
if you have say 10x the average hourly trading volume u will start to have a lot of problems trying to get in or out quickly. if you have half or less then it shouldn't be too bad.
this is just a rough idea how to figure it out for yourself.