Ah, thanks. If I get a good card (say Nymph), and I don't visit the Oracle the next day, will my starting cards change anyway?
You can keep using the same card for a few days(whether you visit the oracle or not) by just leaving the deck as it is or by using modify deck rather than build a new one. However your deck will start losing hp every day. For the first few days, this is barely noticeable but the amouint lost doubles each day, so by day 5-6 it starts getting to be quite a lot. I would recommend changing your deck based on the following:
Day 1-2: Only replace if your current card is bad and you get a better one to replace it
3-4: This is probably about when most decks should be replaced
5-6: Only if your deck has been doing very well and is still winning despite the hp loss or if you had gotten especially terrible cards the previous couple days
7+: Your deck was in the top 5 and you feel like being nice and leaving it up as a farm until it drops in rank
Of course almost any card CAN probably be used effectively if you are good enough at deckbuilding and have the right supporting cards available, so which cards you consider good or bad will largely be determined by that. I would say though that most nymphs probably won't actually be that great a card to use(though of course getting one means you have also won that nymph so I wouldn't complain about it either
). Having one or two nymphs in a deck is probably good but with a few possible exceptions, having 5 or 10 of them won't be nearly as good due to the high cost.