Legal Question in Real Estate Law in California

It depends upon whether your ex-roommate was a cotenant with you, or your subtenant with you in the position of her landlord.

If the former, it is a simple matter of a "voluntary, gratuitous deposit" to use the terms of the Civil Code, sections 1813 et seq. The stuff left is a "deposit," she is the "depositor," and you are the "depositary." Formerly, the term was "bailment" rather than "deposit." You must use ordinary care and many not open the boxes while you have duties as a depositary. You may terminate your duties after the agreed holding period by giving reasonable advance notice. If she does not then respond by picking up her boxes, you may dispose of the contents in a commercially-reasonable manner (yard sale, eBay, pawn shop, or if the stuff has no net value, you can dump it) and the net proceeds, after you deduct your expenses, are hers and you must hold the sale proceeds for her.

If, however, your relationship was that of a landlord and (sub)tenant, you come under another provision of the Civil Code - sections 1965 and 1980 et seq.