Am I entitled to demand a share of the profit from my partner in a business venture after three and a half years have passed without the capital being repaid or the amount of profit being known, and what percentage am I entitled to demand if that is my right?

What your partner did is an act of injustice and a breach of trust. Consequently, he is liable for the money you entrusted to him. As for the profit, there is a detailed explanation: if he acknowledges it, he is bound by it; if he is ignorant of its amount, he must be cautious about it on your behalf; and if he denies it, you have no right to it. It is better for both of you to reconcile.