Alternative solution: run the exhaust fan for given time interval rather than opening a window which is easy to automate and serves the purpose of reducing the humidity in the bathroom which otherwise condensates on walls when bringing in cold air from a window in the winter time. A humidistat would be also a helpful device to trigger message.
Another option is to have a echo dot with temperature sensor build-in that triggers message when temperature goes below or above chosen set point. I use this during the summer in the living room to set speed of ceiling fan based on temperature and that works for me very well.

Ok, so this reminder that I want is just not possible with Alexa so easily?

I'm unable to look at the Bosch smarthome app here in Canada so You might be able to do something with it.
 I just re examined Alexa routines and you may be able to do a simple reminder once  the bathroom window is opened wait a specific time then say Bathroom window is open.However doing if else logic is not currently  available with Alexa.

Have tried to disable your phone in the Alexa device list?
You should also beable to uninstall or
disable Alexa in your phone.

You most likely will require a Home automation software. Something like  home- assistant https://www.home-assistant.io/ which also has a Alexa skill and will run on any OS.
I run HomeGenie https://homegenie.it which would do this with some added code for the Bocsh devices.

First of all I would like to apologize. Because my English is not so good, I use a translator.

I have installed some smart home stuff from Bosch Smart Home. Among other things, smart radiator thermostats and also door/window contacts. I have integrated all devices into Alexa and everything works as it should.
The door/window contacts automatically close the thermostats as soon as you open the window for ventilation. However, since with us, for example, after showering times the whole day the windows are then open, it is of course quite cold in the bathroom, especially in winter.
Therefore, I would like to set up a reminder in Alexa, which reminds me or whoever is at home, that the window is still open.
This automation should do the following:
If the window is opened, nothing should happen at first, because that's the way it's supposed to be. After about 30 or 45 minutes, however, Alexa is supposed to remind "Please close the window in the bathroom" until someone actually closes the window. If the window is not closed when prompted, Alexa is supposed to send another reminder.
At the moment, I can only get Alexa to say "Please close the window" as soon as the window is opened. But I want her to say this every 30 minutes as long as the window is open for more than 30 minutes.
And then preferably only between October and April, because in the summer our windows are constantly open, because the heating is off anyway.
Is that possible somehow?