After mhps's comment I dug down into the code and came to the conclusion mhps was right. That is, a 1000 day old chunk of 100 coins will get get 1000 days worth of interest. And there is no limit to the number of days. If you don't open your client for 10 years, and YACoin is still around in 10 years, then you will get 10 years worth of interest.
Thanks for taking time to confirm. Just a comment -- the good thing about giving you 10 years' worth of interest if you open your wallet once in 10 years is that people are attracted by the easy interest and become a long term user of the coin; the bad thing about it is that the savers would tend to open wallet once in a long while hence lending little support to the network through being a POS minting node. There are some relevant discussions here in the contect of PPC
https://bitcointalk.org/index.php?topic=127954.msg1360154#msg1360154