You can calculate your approximate storage requirements by using manufactures published data. BUT you should never use a single storage card except in cases when you also have backup copies whose integrity is guaranteed. A minimum (but incomplete) requirement to guarantee (in normal terms) the integrity of a backup copy is to have 2 backup copies (in addition to the master copy).
All data storage devices fail.
If you reply on a single copy of data on any media you can consider not having lost it yet a matter of good luck.
Hard disks fail somewhere between 1 hour to 10 years from new, in most cases.
I've seen examples of both, and "a few years" is the somewhat safe limit to "trust" a hard disk for, as long as it is fully backed up very often.
SD cards MAY last decades without failure in some cases. But in most cases a card that is regularly used and never formated can be expected to run perhaps tens of load/download cycles without fault but would be exceptionally lucky to do so for hundreds of cycles.
If you format the card no less often than say every 5 load download cycles it MAY run for far longer without faulting. But, may not.
But, if the best quality USB memory card made by a top manufacturer corrupted a single file on any occasion that you used it, or lost much of the current data but was able to be reformated, or failed permanently, then nobody should be in any way surprised. While very low price and unknown brand is an indicator that you are more liable to have problems, and while high price and top brand indicate that you will probably not get worst case results, neither brand or price is an assurance that you will not lose some or all your data on any given day.
Never rely on one only card if a card failure would be catastrophic for you.
Always keep at least two and possible 3 copies of all files that you do not wish to ever lose.
If data loss matters to you swap out a card containing valuable files as soon as possible and replicate it as soon as possible. (When I am taking photos at events etc where file loss is completely unacceptable I rotate through several cards, download to a backup device (incrementally to an HP netbook usually in my case) during the current "session" and also retain the photos on the card at least until I have copied the files to a 3rd device (in my case a portable USB 2.5" hard drive).