As with the question of ultimate success, there are so many individual variables that it is really hard to say for sure - but many people do experience substantial improvement in a first lesson!

Some years ago I gave taster sessions on a stall at a health fair - all day long everyone who had a 20 minute taster got instant benefit!       And then some long term and very committed clients have progressed much more slowly - for identifiable reasons in most cases.

I am surprised, though not worried, if someone doesn't experience anything at all in a first session but improvement doesn't always come in the form expected.

Someone who had experienced endless sleepless nights from eyestrain came to a second lesson, adamant that 'nothing had changes' until I asked how she was sleeping?
'Perfectly well - why do you ask? What's that got to do with it? Oh!'

The Bates Method is very much about learning to relax:
generally speaking if that starts with learning not to worry about how fast you're improving, it will happen sooner than you think!