When air-crash investigation completes, either NTSB/FAR/other investigating agency finds some issue with instruments/equipments. What is the minimum timeline given to instrument/equipment manufacturer to fix the issue? (May be based on the severity of the issue).
That is up to the civil aviation authority, which in most countries is distinct agency from the investigation board (in USA this is FAA), to decide.
They will take into consideration how likely the problem is to repeat, how big risk it would be if it does¹ and as secondary criteria how much work the fix is and how much it will cost. What must be done will be published in an airworthiness directive.
If the problem is serious, they will prohibit any further flights except for testing purposes until fix is installed. And remember the fix may not be known yet. For example after the 787 battery fires, FAA published airworthiness directive that prohibited operations until “approved fix is installed” couple of days after the third incident when they just started to discuss what the fix should be with Boeing and whole fleet remained grounded for almost half a year until the modified batteries were ready.
On the other end of the scale a problem may not necessarily result in mandatory change at all. Many issues simply result in mandating additional inspections, shortening intervals between inspections or shortening maximum service life of some component, which may be relaxed again if the manufacturer develops an improved version of the component and the operator installs it, neither of which is mandatory. Even quite severe problems may just be solved with inspections, because most components have inherently limited life and the problem just indicated higher wear then originally anticipated.
¹ It is important to remember that the investigators don't only investigate crashes, but they get reports of all incidents, which means any failure where there was elevated risk. They then investigate those where they consider the risk significant or that are more common then expected. So often things are investigated where the probability of fatal outcome is low. In such cases the investigators will only consider safety and will publish a safety recommendation, but the aviation authority has to also consider other practical aspects and may choose not to enforce it.
It would depends on various factors.
First is the severity of the issue. If it can cause a accident then the entire fleet of affected aircraft will be grounded until the fix has been certified and installed.
If the issue is not that severe then it will depend on how hard the fix is to install. The harder/more time consuming it is the more time is given. Some fixes can be put in during the normal pre-flight check.
Then they will give enough time for the fleet to get cycled through the scheduled more extensive A or B checks where the fix will get installed.