What if I cant have the wallet running 24/7, how much inpact if I open the wallet lets say for 3 hours once every week ?
It lowers your opportunity to stake. Wallets need to communicate with the network to determine coin stake age, and probabilities are computed to see if coins will stake. Plus, once they stake they have to be communicated to the network or they will become orphans.
You can do this - once a week - but I would give a wallet a full 24 hours. It should not change your stake return perceptibly.