How long is a piece of string?
Seriously, there could be no definitive answer on the basis of the information you give us.
So, first off : can you avoid the d-checks? If your aircraft are leased, it makes great sense to turn them back in prior to d-checks and replace them with newer planes with longer checks on them.
If you own the aircraft, then you should consider manually doing some of the checks early, to spread out the down-time and avoid having two aircraft in the hangar at the same time : why? Cash-flow is why : while the aircraft are off-line, they still have weekly costs, wages mainly, their share of back-office costs, like marketing, etc : and you get no revenue : so, having all your aircraft go down at the same time will hurt your cash-flow badly.
Your 5 aircraft having 60 days each down is a lot of revenue to be losing : consider if its worthwhile short-leasing a spare plane to cover their routes, switching schedules as each falls due, and so keep that revenue flowing : perhaps you might then keep that spare and open up new routes, if demand exists.
Another way to minimise revenue loss is to rotate the schedules on the aircraft so that your weakest set of routes are the ones 'off' whilst keeping your most profitable sets flying.
Will it kill your airline? No, not if you plan ahead for it and seek to keep maximum revenue coming in to cover the costs of the checks and pay your regular bills. Dont forget, any loans will need their repayments covered too, miss one of them and that will certainly hurt your airlines credit-rating for some considerable time.
A further consideration is that time off the schedules will allow any competitor to consolidate their position, you will have to claw back your previous market-share once you are flying them again.
So, the 'correct' path does depend, as always, on the precise conditions of your airline, routes, and competition.
Plan for it, and its a blip on the oath to greatness : let it just 'happen' and it may well cripple your progress, if not finish you off.