Leads will stay Leads indefinitely until you take some action to either turn them into opportunities or mark them as lost/dead. You will notice at the top left of your form there is a button labeled Convert to Opportunity. At any point, you can convert a lead into an opportunity simply by clicking this button:

Once you click Convert to Opportunity, you will get presented with an Odoo wizard that will allow you to choose how you wish to handle converting the lead into an opportunity:

Each of the options presented are pretty well self-explanatory. The conversion action determines whether you will create a new opportunity or merge this lead with an existing opportunity. You get the option of assigning the opportunity to a specific sales person.

Finally, you get to tell Odoo if you wish to create a new customer for this opportunity or if instead you wish to assign this opportunity to an existing customer.