What is considered "too long" for taking an nsaid that it would cause stomach problems?

Varies: I suggest ALWAYS taking NSAIDS with Zantac (ranitidine) (H2 blocker) or Pepcid/Nexium (PPI therapy) for anything more than 3 days of straight NSAID therapy.

Any time : The stomach (kidney too) is always suffering a degree of injury from NSAIDs; but for some, because of a range of factors (like health history, age, meds) the extent of damage (like ulceration, bleeding) & symptoms (like pain, loss of appetite) is greater than for others. Less is better. If need once-twice daily for more than 2weeks, then discuss with your primary medical provider or specialist.

No set time duration: NSAIDS can cause problems such as gastritis or stomach ulceration even after relatively brief periods of usage. NSAIDS have a black box warning indicating increased risk of stroke or heart attack. Cardiovascular risk may be increased in patients with other risk factors for CV disease, such as smoking or family history or Diabetes. Don't take NSAIDS on a chronic basis unless your physician ok's it