Any idea when this is likely to start? 

Is medicnal kratom now legal in Thailand?  I thought it was only medicinal marijuana (spelling) that has been legalised.

Both, but I wouldn't hold my breath (ptp) for the legal procurement of either. These decriminalization moves were purely business decisions so that they wouldn't be left behind on the development of pharmaceuticals, though that's already happened on the marijuana front. Kratom has been available in the west for years, a lot of it shipped from here. I saw it in an Edmonton shop 10 yrs ago and I've read that in certain states it can be had very openly (though the DEA tried to get it banned). There's an old Thai saying apparently 100 yrs old paraphrased as ' rather have my son-in-law chewing tom than smoking ganja (tom chewer being the harder worker -  a productivity issue apparently-lol).  Now, they may get a jump on kratom potentials though I suspect that they're behind in that development also for it has many interesting properties. IMHO the only real benefit Thailand would ever see money wise is if they legalized rather than decriminalized. Sadly, the PM for life has announced it will never be legalized for recreation on his watch (or was that Prawit lol) so I don't expect to see tourists blazing up in my lifetime. 

It will take some time as will medical cannabis. All that has happened (and I think not yet fully in effect) is that these have been moved from Category 5 narcotic (completely illegal) to category 2 (legal for controlled medical use  - same category as morphine). This is an essential first step but more has to happen before it is available. There are as yet no Thai FDA registered products nor are all the controls clear but as they are category 2 narcotics it is a safe guess it will be hospital pharmacy only with records of sales reported to the FDA.