How long must the capital stay in the bank? Can I withdraw it right away?

The process is quick: on day one you open the preparatory account and deposit the capital; the next day you can request a deposit balance certificate. Once you have the certificate, the funds can in principle be used (a few banks have their own withdrawal rules; follow the bank).

But please note: do not withdraw everything the moment verification is done. The capital is the company's operating money. If you are later audited and cannot produce vouchers showing the money was used for the company, it may be deemed "false capital", and the penalties under Article 9 of the Company Act are severe. The normal approach is to keep the money in the company account and spend it gradually as operations require (rent, purchases, salaries), keeping the vouchers.