I'm sure some of you been here long enough to answer this for me but does every game in the GOG catalog go on sale over the course of a year or are there some that don't go on sale? Do some go on sale more than once?
I'm guessing the answer to all those is yes but my reason for asking is to get a better idea of how sales work around here.

Reason I ask is I only buy games anymore when they are on some kind of big sale (usually 75% but 50% works if its a game I really want). With the Activision sale this week for example, will these not be on sale again for a very long time and even if thats the case, any chance there will be a bigger sale on them like a summer sale or holiday sale like Steam has? Not trying to be cheap if anyone is thinking that but more to getting best value for my money since the library here is extensive.

Every Monday and Wednesday a single game goes on sale. Every weekend is a bigger sale, and GOG usually does winter and summer sales, which is pretty much every game on sale. Periodically they might throw a bigger sale up, like all D&D games since those don't go on sale often.

So yes, some games do go on sale more often than others.

Everything goes on sale but there is one underlying rule to that: As per GOG policy a game is to go on sale ONLY AFTER you have just purchased it.

I submit Settlers 3 last week and I know a few people got nailed with some adventure titles today.

Heh, that's so true. =P

Most of the catalogue goes on sale during Christmas (at least it has for the last 2 years). There is a sale on summer as well, but no set pattern.
As for the sales, we have 2 gem promos per week, one Classic title discounted on Mondays and one Indie/newer title discounted on Wednesdays. As for the weekend promos, it varies. Last 2 months we had genre promos and publisher promos, while we have also seen previously theme promos (Nordic Countries for example).
There are a few games that rarely go on promo, but Activision should be discounted again later this year, though probably in 6 month time.
For the last year or so, I think 50% is the minimum discount, with 60% being the more usual one, and 75% the more rare. We have had some weirder discounts as well, including a "Pick 5 pay $10" promotion.

So, the usual advice is: If you are going to play it now (or soonish), buy it now, else you can afford to wait.

sorry mate no offence intended but buying games only on sale is a bit cheapassing. plenty of good games at launch deserved to be bought. anyway, no way to tell with gog.the well known publishers and games usually go on sale once a year at least.the rest really is random.biggest sale is around Xmas time.even then not all games go on sale.if u see something on sale generally, just grab it then.