You can at the very minimum, lose shades of between 1-2. I'm assuming you've used lightening creams or lightened your complexion in the past of course, so you're very likely to lose even more, if not 4-5. I'd go for it of course. Alpha Arbutin is a lot cheaper than most other lightening creams, and is a great skin maintainer, preventing major melanogenesis through it's inhibitory processes.
As for stretchmarks or ochronosis, Alpha Arbutin is a good combatant towards removing stretchmarks, so the chances of it causing more is quite unlikely. Alpha Arbutin doesn't seem to have any relation to excesses in homogentisic acid nor metabolic acids, therefore I doubt it would cause ochronosis either.

