Left untreated, the memory loss can last for as long as any individual player wants it to. Feel free to make up a reason your character has it for a longer or shorter time - brain chemistry, mental shielding, magic, etc. can all be reasons.

As for talking to the plants - yes, the talking plants will give tips and hints on what plants in the shop and/or in the Agricultural area might be combined to create a cure. They won't give a direct answer (mostly because they're trolls with twisted senses of humor) but will give characters some ideas.