My dieffenbachia has started to look rather sad over the past couple of weeks. First these pale brown spots appeared on the leaves, and then one by one the leaves have started to yellow, droop and then die. It looked healthy before this, I have not moved it or changed watering habits. Any advice to prevent further damage would be greatly appreciated.

Check the undersides of the leaves for spider mites that look like tiny particles of dust.

How do you decide when to water and how much do you give it? How often is that?

How long have you had it and has it ever been repotted?

There doesn't look to be any evidence of spider mites, the undersides look pretty clean.
I water it about twice a week, or when the soil looks dry. Does this sound about right? I usually water it under the water starts to drain through the bottom.
I've had it about 7 months, and repotted it yesterday in the hopes it might make a difference

You have been overwatering it. It should not need a thorough watering more than once per week. The frequent watering has not allowed the soil to dry out properly and for the roots to get the oxygen they need.

Unfortunately, repotting is rarely a solution for any plant problem and often creates new problems.

How much of the original soil did you remove when you repotted? In the photos you posted, is it in its new or its old pot?

Thanks for the reply!

The photos are post-repotting. I kept most of the previous soil and moved it into a slightly bigger pot.

It can often look a bit droopy and seemed quite reactive to watering so that's why I was leaning towards giving it more rather than less, but I'll start giving it less and see if that makes a difference. The soil didn't feel too wet when I was repotting

Remove the soil you added to the top of the original rootball so that the uppermost roots are just barely covered. Then wait for the top half-inch of the remaining soil to feel dry to your touch before watering thoroughly.

It appears that it is mostly the older larger leaves that are declining. The good news is that new growth appears to be healthy.

I neglected to ask previously how far you keep it from the nearest uncovered window.

Top soil removed and I'll keep an eye on it before watering, thanks!

Yes it's still growing new leaves but some of the bigger (albeit) older leaves are yellowing which was a concern.

It's about 2 meters from a large SE facing window, does that sound ok or should it be placed in more of a sun spot?

Move it closer to the window but just beyond the reach of the sun's direct rays.