How heavy can tiles be on a partition wall?

To assist with structural performance the wall should be properly over-clad with quality tile backer boards in accordance with the backer board manufacturer's specification to suit the intended tile. Then a suitable high quality adhesive should be used to ensure proper adhesion. Followed by good and effective grouting. Essentially there is no point taking short cuts - quality products, fitted well will produce a lasting and high quality job.

when you take the tiles off it wouldn’t surprise me that the plasterboard comes with it in places if done properly in the first place. I would remove tiles, maybe replace plasterboard or add screws centred to where nails are if the plasterboards in good condition, good adhesive with correct application (not dot and dabbed), tile backing board would be good applied with grab adhesive over plasterboard secured with screws and washers and tanked if you like? And tile. If your really worried for peace of mind you could increase the strength of the stud itself! Try not to put your back out!

Around a bath you dont want just plasteboard. Use a backer as already stated. Go with ditra schultra uncoupling membraine. If you are concerned or think the tiles will be extra heavy, prime the plasterboard first with either a proper primer of some 30% pva to water slosh, as this process will add strength to the subrstrate. The point of a membraine is it protects from cracking and creates a waterproof barrier behind tiles to sheild wall from moisture