My company allows me to purchase stock at 90% of the closing fair market value. My salary is in the 90k-100k range. I am allowed to put in anywhere from 1-15% of my paycheck in company stock up to a max of 25,000$. I have just begun employment at this company and my age is in the lower 20s. Since I just started out and I am single with no dependents (for at least 2 more years), it seems like I have more money on hand than I will have in the future. What approximate percentage of pay should I invest?
EDIT: I am referring to ESPP, the company is in the US, is publicly traded and the stock is known for being quite stable.