Asynchronous data fetching and cache handling

Question

I'm planning on deploying a dynamic site that needs certain tasks to be done periodically in the background, let's say every hour or two. The data that i need to output is strictly depending on the result of those queries.

Now, the problem is that those tasks require a certain amount of time, e.g. 8-10 minutes: so if a user asks for a page, and therefore a certain view gets called, and that task is still in process everything goes bad.

So, here's the question: what do i need to implement to handle that problem? I've been searching and i found about celery but i haven't understood how it could be used to fix the above issue and maybe implement a caching system that intervenes when the tasks aren't finished yet.