What Is Gemini 2.5 Pro’s Effective Context Window?

Gemini 2.5 Pro has around one million tokens of context window. That's huge. But what is its effective context window, or how much can it handle before hallucinating or being unusable?

I know that specific or good prompting also matters how the model answers and how you manage your prompts, like deleting or editing them before sending another prompt along with what you use them for, like coding, roleplaying, design, translation, and etc. I would very much like to hear your opinions on this and your experiences.

My normal sessions with Gemini 2.5 Pro only comes to about less than 90k tokens, so I have no knowledge how the model behaves in larger context window.