Why clustering? It doesn't look like clustering problem. You can make cluster analysis as preprocessing phase to distinguish several groups of users (or you may omit this phase), but then you need to do some kind of numeric prediction: both - count of installments and days - are numbers, so how are you going to get these numbers with clustering?
I suggest you using regression for this task. Linear regression must fit your needs. If dependent variables (# of installments and days) depend on other attributes non-linearly, you can try polynomial regression or even algorithms like M5', that first build decision tree and then add regression model to each leaf of that tree.
If you have non-numeric attributes, you can also try to use classification - in this case you need to manually create possible classes (e.g. # of installments: from 3 to 5, from 6 to 10, etc.) and then use any of classification algorithms (C4.5, SVM, Naive Bayes to mention a few).
Actually, I don't think you have tons of data. I believe if is less then 50Mb overall, so there's no need to use monsters like Mahout, that are designed to process really, really big amounts of data. You can use Weka or RapidMiner for this purpose. Even if they are not able to handle your data with default config, just increase memory for JVM and in 99% of cases they will be ok.