I've had good results with Gyokuro cold-brew (created overnight in the fridge).
Noticed a much improved ability to concentrate on mundane tasks, and much less procrastination. I think, effect lasted for some 6-8 weeks, after that got off by drinking coffee during the day as well, so not sure what actually stopped the effect - caffeine or l-theanine tolerance.
I only need to not get too high on caffeine and to not repeat the same tea so much, if I do when I realise and switch to the other one the effect comes back.
Getting Oolong's and senchas with good theanine content is hard, specially finding one that works for you. Glutamate rich smell is a good indicator as well as the color or whiteish spots. Gyokuro or matcha is a safer source but if you only use that you get the tolerance thing too.
Yellow bag Lipton also works a little bit for me but not as effectively.
As gp said, 1 espresso, 6 hours.
For me it also cuts down the side effects, all around a very useful substance.
During a regular onsite interview, without l-theanine I might have some heart racing, and I might not be able to focus well enough to even understand what an interviewer is asking me. With l-theanine, the anxiety is still there but I can at least understand the problems I'm given.
I think I can taste the difference in sodas.