I am thinking about getting into magic. I want to play in the standard format. The decks I want to play are either mono black devotion or mono blue devotion. My question is with the rotations how long will these decks be playable? I don't want to spend the money and then have a deck I can't play.
Devotion came out in the Theros block so any card with "Devotion to" will be in standard until about fall 2015 when the Theros block cards will no longer be playable in standard.
Some of the cards you will be getting for these decks will have been printed in the Return to Ravnica block (Pack rat, Nightveil Specter) and you will only be able to play these in standard till fall 2014 (whenever the set after M15 comes out).
Without knowing what cards will come out in the fall 2014 set, these decks may either still be around, dominate all the tournaments or just drop off the radar once the cards from Return to Ravnica block drop out of standard. Standard is always about investing into new cards every year.
The cards you do buy can be used for either Modern or for trading off for newer cards so investment is never fully lost, you just may never get what you paid (but my guess is the Gods will still have value after standard since they can also be used for commander)
If you're playing in Standard tournaments, you're almost guaranteed to have cards rotate out of your deck(s) every October. The only way to avoid the 'problem' is to replace older cards in your deck with newer cards before rotation happens... which still has you spending money to keep your deck up-to-date, it's just a matter of when you make the changes.
An alternative would be to get into Modern, Legacy, or Vintage formats instead of Standard. While the three non-rotating tournament formats generally have a higher initial price to get into, the lack of rotation means that once you've created your deck, you'll likely be viable for as long as you want to play.
As new cards get introduced, Modern/Legacy/Vintage decks occasionally still need updating, it's just not because the deck lost cards; rather, the deck needs to be updated to stay competitive when new threats or answers are introduced. For example, with the introduction of cards like Deathrite Shaman and Rest in Peace, Dredge decks have to be more careful about sideboard (or even mainboard!) hate.
I have also seen a number of people who start playing Standard, and as their cards rotate out, they build up Modern decks from the ex-Standard cards. This doesn't avoid the 'problem' of updating your Standard deck(s), but it does keep most of your cards you've spent money on useful. Or, you could simply sell the cards shortly before rotation to try and recoup some of your losses. (And if the card that's about to rotate has become big in a non-rotating format, you may not even end up losing money at all!)
Another option would be to use the cards that have rotated out of your Standard decks to build casual decks, such as for EDH/Commander.