In my opinion it's not a question of "how long before Pakistan can host international cricket?" - the first question is "what does Pakistan need to do to be able to host international cricket?".
It's not as if we are serving some fixed term punishment or exclusion which will come to an end, and we will suddenly find ourselves in a position to host international cricket again in 6 months, a year or even 10 years. It's simply a case of addressing the fact that cricket was targeted by terrorist groups within Pakistan & what does Pakistan need to do to assure visiting teams that they are safe in the country going forward.
Sure - their have been terrorist attacks in other parts of the world which still host cricket matches, such as England, India & Bangladesh - however, the big difference here is that cricket and cricketers were never targeted in any of these attacks. The fact that cricketers have been targeted even once in a terror attack within Pakistan, is enough to ring major alarm bells for cricket boards around the world before agreeing to send their team over.
Now in my opinion the only way to assure international teams that cricket will not be targeted by terrorist groups is to eradicate terrorism completely, which is obviously a lot easier said then done.
Their have been 19 terrorist attacks in Pakistan this year which has resulted in the death of approximately 300 people. Now if you are talking about a country where a terrorist attack is taking place - on average - every 10 days, approximately 2 people dying every day due to terrorist attacks and in the past terrorists have targeted international cricketers - it becomes very difficult to assure touring nations that they will be safe in such situations.
Many on this forum know that I am ever optimistic about everything Pakistani cricket related, however, the harsh truth is that we cannot close down a city every time a cricket match is going on, like we had to do in Lahore when Zimbabwe were touring. I would absolutely love to see international cricket played in Pakistan again, however, we have to be sensible and realistic with our expectations.
To a certain extent I think certain parties are trying to exclude Pakistani cricket (e.g. BCCI refusing to send their team to UAE to play VS Pakistan, not having Pakistani players in the IPL etc.) - however, when it comes to not being able to host international cricket in Pakistan I don't think we can blame anyone else. We are the ones who have gotten ourselves into this mess, and only we can (and InshAllah we will) get ourselves out of this mess. You cannot expect any country to risk sending their team to Pakistan giving the situation we currently find ourselves in.
We can wait for 6 months, 1 year, 5 years, 10 years, 20 years - however long you want. The bottom line is, until or unless we eliminate terrorism from Pakistan, we can forget about hosting international cricket. This will of course be no easy task, specially given that we currently - extremely unfortunately - live in a world where idiots are carrying out such cowardly acts, more and more frequently every day.
In my opinion the most important question is "how long before we can eradicate terrorism, terrorist activities and these terrorist attacks from Pakistan?". Once we have an answer to that question (which for the record IMO is a much more important question to answer and resolve ASAP), we will know exactly how long it will be before international cricket returns to Pakistan on a regular basis.