いつまでも means forever, for a very long time, or indefinitely. It is almost always written in hiragana. The kanji form 何時までも（いつまでも） exists, but it feels uncommon and literary.
The tone depends on context.
In warm or sentimental contexts, いつまでも suggests something lasting no matter how much time passes, such as いつまでも一緒（いつまでもいっしょ）, “together forever,” or いつまでも忘れない（いつまでもわすれない）, “never forget.”
In critical contexts, it can sound impatient: “How long are you going to keep doing that?” It is often used when someone is dragging something out, dwelling on a problem, or not moving on.