Is there a period of time for condition decrease?

Hi Chlorinated,
There is no official time period. However, I think it unlikely that unless something drastic happens such as amputation, or there's an accident etc it is unlikely that DWP will accept a worsening condition that has recently occurred. That said, by the time you realise things have become worse, especially if this isn't a dramatic change, then you have obtained a new form, filled it out and returned it, then and possibly had another assessment this could easily be several months!
Remember that if you are already on enhanced daily living and/or mobility, no amount of worsening will give you any more benefit for either component. Likewise if you already score the maximum amounts under any one descriptor, or the maximum that you would ever get for your kind of disability, again you will not achieve further points, despite eg more pain, more lack of movement etc etc.(Thus is because PIP points can be awarded when you can not perform the activity for the majority of days, so if your previous status allowed better days, and now you just have bad days, more points cam not be awarded.)
However, if you don't have the enhanced or maybe even standard rate on either section then of course worsening disability such as less walking ability could provide more points and allow benefit or a higher award as appropriate, so it would be worthwhile applying to DWP, who will then review your whole claim. Whilst often agreeing that more points should be awarded, DWP may decide to award less. It is unlikely but does sometimes happen!
It is unlawful not to report an improvement in your health where this would result in a reduction in benefit.Clearly if you are no longer entitled to all or part of your previous PIP not only will you be overpaid, but likely to be investigated for fraud!
DWP ask for you to report worsening conditions, so that you can get more benefit if appropriate. Clearly you are not going to be committing fraud if you don't bother to claim a higher amount of benefit that you deserve!
I advise my personal clients not to inform DWP of worsening symptoms unless you want to claim more benefits. Clients have never come back to me saying this has been a problem. The only caveat is that if you go into hospital, or a care home this can affect your benefit after 28 days, so you must disclose this.
As I said earlier, without a drastic incident, conditions often become worse over time. You will find yourself thinking one day, oooh this is very difficult today, I used to be able to do this easily. Two or three days later you think the same and realise things have got worse, and then you will probably look back and conclude it could have been several months ago that you did the activity better but you just haven't realised!
Also bear in mind that some people will have flare ups. Initially you might suspect a flare up, and will genuinely believe things will settle down on a week or two. A month or so later, and things haven't settled you could probably rightfully conclude you have actually got worse and it isn't a flare up. It is of course difficult to know until time has passed!
I hope this helps.
LL26