Size GPU duration to measured timings

app.py

@@ -19,6 +19,7 @@ sys.path.insert(0, APP_DIR)
 import spaces  # noqa: E402  (before torch)
 
 import json  # noqa: E402
+import math  # noqa: E402
 import random  # noqa: E402
 import tempfile  # noqa: E402
 import time  # noqa: E402
@@ -120,7 +121,11 @@ def proxy_video(name: str, chunks: int) -> str:
 
 
 def _duration(scene, chunks=8, seed=0, *args, **kwargs):
-    return int(40 + 14 * int(chunks))
+    """Measured on ZeroGPU xlarge: ~4-6.5 s per chunk while the 6-chunk span fills, 7.6 s once it is full,
+    ~1.7 s per chunk of VAE decode + mp4, ~8 s fixed; +15%."""
+    n = max(1, min(MAX_CHUNKS, int(chunks)))
+    stream = sum(4.0 + 0.5 * k if k < 6 else 7.7 for k in range(n))
+    return int(math.ceil(1.15 * (8 + stream + 1.7 * n)))
 
 
 # --------------------------------------------------------------------------------------------- inference