I notice great variability in how this concept is expressed. Does the term vary depending on the context? Should one use one variation when discussing biology, for example, and another when talking about product phases? Wikipedia uses different forms in way that seems random. So, too, does the New York Times, based on basic search. An earlier discussion of the variations in connection with technology notes that "A compound generally starts open (life cycle), migrates to hyphenated (life-cycle), and ends up closed (lifecycle)." Are we at the closed stage for the term in a general sense?
My medical dictionaries (Dorland's Illustrated, 30th Ed.; Saunders Comprehensive Veterinary Dictionary) either list it as two words (Dorland's) or not at all (Saunders). So I would say in medical literature fields, where the context would be something like the life cycle of a parasite, it's still two words.
And I'm not sure that lifecycle and life-cycle are nearly as common as life cycle. Here's the NGram for life cycle vs. lifecycle vs. life-cycle.
Four online dictionaries I consulted (Merriam-Webster, Oxford Dictionaries, Macmillan, and Dictionary.com) all agree on the open form life cycle.
In particular, the Dictionary.com definition in the link uses the open form for all the senses life cycle.
My feeling is that if referring to a single concept, it should be a single or closed word, i.e. lifecycle.
On the other hand, when referring to separate concepts, the open usage would be more appropriate. For example, the various stages of an insect's life would be referred to as a life cycle.
It eventually comes down to context and that is the responsiblity of the individual writer/author/editor, etc.