"This is the longest we've ever spent time together."
- "This is the longest we've ever spent time together."
"This" 「これは」
"the longest ~ ever" 「今まで一番長い」
”we've" = "we have"
"we've spent time together" 「時間一緒に過ごしている」
"This is the longest time ever that we've spent together" も言えます。