It will vary from lender to lender, but a pre.approval is generally valid for 30-90 days; in some cases, up to 6 months.
A pre.approval letter is typically dated with the date of issuance and will state for how long the pre.approval remains in force.
Should you submit a pre.approval letter with an offer (which can give the seller peace of mind that you are actually qualified to enter into the financial obligation you are proposing), be mindful of the date of issuance. Should the seller's agent feel your pre.approval letter is too old, she may likely request an updated letter, which will mean a second review of your credit, verification of employment, verification of deposits, etc.
To avoid having to undergo the process twice in a row and potentially lower your credit score (multiple inquiries into your score can lower it), you might apply for your pre.approval 1-2 months prior to the commencement of your house-hunting. Generally, if you are prepared with all the necessary documentation a lender requires, you can be pre.approved within 24 hours max.
In the meantime, since you do not plan to actually begin your search for at least another year, unless you foresee a huge change in your debt/ income ratio by the time you are ready to buy, there is no reason you could not get pre.approved now just to have an idea of what and where your budget will allow for you purchase when the time does come. By then, enough time will have passed so that a new pre.approval process will not impact your credit score.