To be eligible, during the last 5 years they need to meet the following requirements:
1. Continuous residence requirement: This in general means that there is no trip longer than 6 months each. If there are trips longer than 6 months but shorter than 1 year, they need to provide evidence of proof to ties to the US like paid taxes, mortgage/lease/bank statements etc.
2. Physical presence requirement: They need to be at least 2.5 years (913 days) physically present in the US.
From your post, it looks like your parents could satisfy the physical presence, but given their travel pattern, the officer will most likely doubt the continuous residence and their intend to live in the US. So if they want to naturalize soon, they would need to cut their trips shorter than 6 months each and establish their ties in the US (if they have not already done so). Once they meet both requirements above they can apply.