How long do you have to pray to erase your sins?

Akal Purkh is always present at all the places. Hence, Ardas can be performed at any time and at any place.
ਗਉੜੀ ਸੁਖਮਨੀ ਮਃ ੫ ॥ ਤੂ ਠਾਕੁਰੁ ਤੁਮ ਪਹਿ ਅਰਦਾਸਿ ॥ ਜੀਉ ਪਿੰਡੁ ਸਭੁ ਤੇਰੀ ਰਾਸਿ ॥ ਤੁਮ ਮਾਤ ਪਿਤਾ ਹਮ ਬਾਰਿਕ ਤੇਰੇ ॥ ਤੁਮਰੀ ਕ੍ਰਿਪਾ ਮਹਿ ਸੂਖ ਘਨੇਰੇ ॥ ਕੋਇ ਨ ਜਾਨੈ ਤੁਮਰਾ ਅੰਤੁ ॥ ਊਚੇ ਤੇ ਊਚਾ ਭਗਵੰਤ ॥ ਸਗਲ ਸਮਗ੍ਰੀ ਤੁਮਰੈ ਸੂਤ੍ਰਿ ਧਾਰੀ ॥ ਤੁਮ ਤੇ ਹੋਇ ਸੁ ਆਗਿਆਕਾਰੀ ॥ ਤੁਮਰੀ ਗਤਿ ਮਿਤਿ ਤੁਮ ਹੀ ਜਾਨੀ ॥ ਨਾਨਕ ਦਾਸ ਸਦਾ ਕੁਰਬਾਨੀ ॥੮॥੪॥ (੨੬੮)
However, Ardas should be proper as described in the following Shabad.
ਮੇਰੇ ਸਾਹਿਬ ਤੂੰ ਮੈ ਮਾਣੁ ਨਿਮਾਣੀ ॥ ਅਰਦਾਸਿ ਕਰੀ ਪ੍ਰਭ ਅਪਨੇ ਆਗੈ ਸੁਣਿ ਸੁਣਿ ਜੀਵਾ ਤੇਰੀ ਬਾਣੀ ॥੧॥ ਰਹਾਉ ॥ (੭੪੯)
The purpose of bowing before the Guru Granth Sahib is for the commitment to study, understand and live according to the teachings of Gurbani. Ardas is carried out so that one can lower his ego and prepare himself to listen to the teachings of Guru Granth Sahib. We should beg and learn the way of living by understanding and following Gurbani.
Ardas performed through agents is not acceptable. We have to perform our own Ardas.
Normally we stand before some officer when we are requesting him. Akal Purkh and Guru Granth Sahib are much above any officer. Hence, as a tradition we should stand up for the Ardas. In case someone is not able to stand, There is no such restriction.

One cannot change the past and past deeds. But can definitely modify his present so that future can be improved
ਪੂਰਬ ਲਿਖਿਆ ਸੁ ਮੇਟਣਾ ਨ ਜਾਏ ॥ – 88
We need to improve our deeds at physical and mental level with the help of Gurbani written in Guru Granth Sahib, so as to improve our future. The way some machines can be repaired with proper expertise, same way we can improve our deeds to some extent with True Guru. The decisions and results of our deeds are getting monitored all the time for our every minute action. We keep on getting the results also all the time depending upon the type of action. Every thing gets accounted in this life period only and nothing after death.
The court of Akal Purkh keeps on acting depending upon the Hukam of Akal Purkh.
People have lot of confusion about Hukam. Akal Purkh has created the system covering each and every thing. Everything happens according to the laws of nature applicable both at physical and mental level.
We get the results of our each and every action even at the minutest level. This involves our deeds at physical and mental level both.
ਕਰਮੀ ਆਪੋ ਆਪਣੀ ਕੇ ਨੇੜੈ ਕੇ ਦੂਰਿ ॥ Jap – 8
Most of our actions are linked with the teachings and actions of our parents, family and all types of surroundings. Hence, those all does influence our deeds. In order to reduce such influences, it is always better to follow the Gurbani written in Guru Granth Sahib. We get the results depend upon our deeds.
We can understand the laws applicable at mental level with the help of Gurbani written in Guru Granth Sahib and at physical level with the help of science.
ਹੁਕਮੈ ਅੰਦਰਿ ਸਭੁ ਕੋ ਬਾਹਰਿ ਹੁਕਮ ਨ ਕੋਇ ॥ Jap – 1
Please do not confuse with the deeds of past births. All the results are because of actions and deeds right from the conception to the present.