doing cross fit. How long does this crap normally last?

You're gonna want Aleve and some very low impact mobility, like walking on a flat road.

How did it happen? Too much landing on the toes during plyo stuff?

In addition to Aleve, ice is also your friend.  Tendonitis is inflammation of the tendon.  Make sure to ice it down good after therapy/exercise to reduce that inflammation.

I started to feel it doing front lunge barbells and the next day was a a shit ton of double unders (or a lot more single unders since I have not mastered the doubles yet). It doesn't help that I am old and fat . I've been trying to work through it for 4 weeks now and I keep aggravating it.

Might want to consider one of these:

http://www.kingbrand.com/Patellar-Tendonitis-Treatment.php?matchtype=e&keyword=patellar%20tendonitis&REF=M_ZPatellar_Tendonitis.X

I get Achilles' tendinitis if I run a lot, and the best cure I've found is submerging in ice water for 20-30 seconds every 5 minutes, for an hour a day. Just freeze a bunch of water bottles and get a Rubbermaid bin that allows you to submerge the afflicted area and fill it up in the bathtub with water and drop the frozen water bottles in.

How long does it last?  Forever.  

Got it in my right knee two + years ago.

Tried laying off for awhile, it worked.  Until I started squatting again.

Went to an ortho.  Of course he says, "if it hurts don't do it."  Well, I like Olympic weightlifting and squatting is kinda a thing you do so, how about we fix the problem?  Fucking doctors...….  So, did more direct stretches, exercises and rehab.  Kept it at bay, but it was still there.

Know what finally worked for me?

1.  Stretch after every session.
2.  Ice when you feel like an old fucker.
3.  Straighten your leg out while sitting.  Yup, the 45 degree angle while sitting in an office chair caused enough tension to never let it really heel.  So I stiff leg the right, or stand.

Some other things to try:

Heat - Use a heat pad then stretch
Minimize power movements, do full squatting - Sometimes it's the range of motion, doing full movements might help, or could be the opposite, stop full movements and just power.
Hamstring work - sometimes it's an imbalance, how's your hamstrings and hips?
Direct work - single leg squat at an angle, etc.

Every nagging injury I've had all came down to trial and error, dumb luck or they heeled on their own.

You have an inflamed tendon. You need to rest it until it recovers. Give it a few days before you try even something as minor as stretching.

After you've rested it a bit, foam rolling and stretching both the affected area and the opposing side can help.

NSAIDs will help reduce the inflammation if you're willing to risk their long-term consequences.  Eating an anti-inflammatory diet may also help.