Is l-arginine safe to take long term?

Unknown. L-arginine has been used safely for up to 3 months. There are no long term studies to verify safety for long term use.
Why use it? Why do you want to take it? It is not likely to do you any good. Take a balanced diet and you do not need this supplement.
Visit this site for info in diet: https://www.choosemyplate.gov/
L-Arginine. Long Term dietary supplementation with L-Arginine age-related reduction in renal function, published by Recklholl JF, Kellum JA, Racusum, Hilderbrandt at Dept of Physiology University of Mississippi Med Center in the Am J of Physiology 1997 Jun 272: Art #UI97371364 discusses aging associated with loss of nephron function. It discusses long term dietary sup-Not enough space to detail-