What's the longest it should take to stand up straight after a drainless tummy tuck? I am 2 weeks post op.

Tightening the muscles during a tummy tuck causes some discomfort. A lot of patients tend to protect themselves from pain by bending at the waist. So, what you are experiencing is somewhat normal.
My first bit of advice is for you to discuss this issue with your doctor and formulate a game plan with him. It does sound like you had a drainless tummy tuck with absorbable progressive tension sutures (PTS). One of the points of using this technique is that the absorbable sutures secure the abdominal skin to the muscle at multiple points. This distributes the tension of the tight skin closure away from the incision and to all the suture points. So, most plastic surgeons who use a PTS technique advise their patients to begin to straighten up the day of surgery. The goal should be to be walking straight by the end of one week. 
You shouldn't wait until the sutures dissolve because this is a 3 month process and you will definitely have difficulty straightening up after 3 months.
So again, talk to your doctor and ask him about his philosophy of straightening up with PTS sutures. 
Good Luck!

Healing after a  #tummytuck will require time. Some patients are out of bed and walking the night of surgery and every hour while awake. I allow my patients to return to work at one to two weeks with 14 days preferred.  However, no lifting or straining.  At three weeks, increased level of activity and full with no restrictions, at 6 weeks. As I advise my patients, if your work keeps you sedentary, you may return whenever you feel up to it. If your work is strenuous, wait until your work activity does not cause any superficial pain. Some employers will modify a person’s job duties so they can back sooner, but without physical activity. Our office can provide our patients with a note stating they are not to engage in strenuous activity for a specified period of time. The note will not disclose what procedure they have underwent. Therefore, you may have to request a similar note from your surgeon if you are not independently employed. I recommend avoiding heavy lifting and strenuous #activity for six weeks following your surgery. With that said, it may help you plan your return to work accordingly.As for heavy lifting and other #strenuous #activities, it should be avoided for until you have clearance from your surgeon. You may, however, do normal activities at any time if they cause no pain or #discomfort. Let your body tell you what you can or cannot do. Aerobic exercise will raise your blood pressure, which could cause late bleeding and harm your result. Once you begin exercising again, start gently and let your body tell you what it can tolerate. Don’t rush!!  If you have concerns about your healing, or pain that you question to be unusual, it is important to call your plastic surgeon to discuss these further asked to be examined.

Drainless tummy tuck has been a wonderful addition to my practice as it adds the advantage of getting patients moving much faster!  Drains are painful and confining.  I generally advise my patients to allow their hips to flex and stay slightly bent over for two days, then I coach them to start standing up after the second day.  Each time that one gets out of the chair, or out of bed, or off the couch, then the straightening process must begin again.  However, the sooner the patient gets straightened up, the sooner the hunching stops.  If the hunching continues for more than a few days, then lower back pain becomes a huge problem.