Basically, the Explorative Test should be planned accordingly over a Timebox. But how do you define these? Does the team decide within the sprint planning how big the effort for the exploratory test is? How do you rate this within your teams?
Ranjeet listed pretty much every point, my 5 cents :
First priority should be given to Business Value, followed by security vulnerability, complexity and usability.
But one think to note is, QA's should never negotiate on the exploratory effort. Exploratory testing is what helps a good tester show his value to the team. Developers and product owners will be checking mostly the standard behaviors of the application against a set of expectations and this is where a good tester can standout and show his/her real capability. Coming up with scenarios which others might not have thought about, will make the application effective.
If as a QA you are spending lots of time around regression or sanity testing, automate those monotonous testing activities and free up enough time for the exploratory testing.
It depends on the Sprint workload, complexity of application-under-test, size of the testing team and their experience. In my last project we used to give qa folks 2 hours everyday before 5 days of going live. In my current project, we give 1 hour each day for last 5-6 days of sprint(3 week). But again it varies from project to project.
In our teams exploratory testing is part of the user stories definition of done. We estimate user stories with relative complexity to each other. Thus more complex stories probably have more room for longer timeboxed testing sessions. We set a timebox, after the timebox the teams discusses if another timebox is necessary, depending on the found issues or risks.
Personally I really dislike more detailed planning, certainly if these details contain hours. It is also irrelevant if the full-teams focuses on getting stories DoneDone and into production, exploratory testing is just part of the work.
If you do handovers to QA roles and or other resources, you might feel the need to have a more detailed plan, but most often the plan is just a bunch of lies. I like teams that swarm their user stories, exploratory testing is an activity that anyone in the team should be able todo complete the story.
Suggested time box for Exploratory Testing(ET) is between 30 minutes and 2 hours. When I use ET I set the time box 25-30 minutes for exploratory and 10minutes to go through my notes and make them readable for others (or myself in two weeks). Sometimes I write small report - how many ideas I explored during session, what caught my attention, what I want to test next.
"Does the team decide within the sprint planning how big the effort for the exploratory test is?" This needs to be decided in team, what is important to investigate. It could be based on risk, business value, usage etc. Important: when you write Test Charta set as aim something small - one aspect of your feature, never whole task or user story.