Is there any schedule for sales on champions?

I've been playing with Ahri for the past few days (she is on the free-to-play rotation this week), and I really like the way she plays. Currently, she costs 6300IP which may not seem like a lot to players who've been playing for a long time and have accumulated a great deal of IP, but in my case that is a lot. I'm just below seven-thousand IP, but I just want to know if there is a schedule of any kind for sales.

In short: is there any way for me to tell if Ahri (or other champions) will go on sale? or am I better off just buying her instead of holding my breath?

I know that older champions get cheaper over time, but I'm not sure when Ahri was released. Also I looked at this, but I feel as though it doesn't address what I'm looking for.

Note: I'm only spending with IP not RP.

Sales are always announced the day before it starts. So other than that there is no warning.

Here is a list of previous sales. Searching for Ahri you can see she was somewhat recently on sale (April 5-8) which means she probably won't be again for a while.

This google docs spreadsheet is completely fan-made and theoretical, but predicts what future sales could be based on how long it's been since a champion has last been for sale. Can be a bit useful but remember, it's not official at all.

When there are sales, only the RP price reduces and not the IP price.

The current Riot IP pricing policy is, that the latest release is 7800 IP for a week before dropping to 6300 IP. Concurrently with the release of a new champion, the oldest one at 6300 IP gets reduced to 4800 IP.

As an example, with the Vel'Koz release they have reduced the cost of Shyvana to 4800 IP. You can sort the list of champions on LoLKing by release date to see which is the oldest champion who is next in line for a reduction.

Re. Ahri, she is 3rd in line after Fizz and Volibear. Lately champion releases have been more infrequent, if we assume that it takes 2 months on average between new champions then she should be free in another 6 months (from Vel'Koz release). Of course, this is highly variable depending on what Riot has in the pipeline.