MSE did the calculations a while ago and has a calculator on the site which works out your odds. https://www.moneysavingexpert.com/savings/premium-bonds/
Basically your chances of winning, say, £10k are 1 in 1 in 3,352,821 per each month draw but that is per £25 'bond.'
With average luck, someone with £10k invested will win £375 per year.
As a pp said, if you want a definite 'win' put it in an ISA instead. Obviously you can only put in £20k per year, but if you top it up again this and then next April, if interest rates stay the same you'll get £2600 per year guaranteed (based on current highest interest rates of 5.2% on £50k, compared to an 'average luck' win of £1,950 on premium bonds).
Of course with higher than average luck you could win a million. But with lower you could win nothing at all and be £2600 down.
I only put money in premium bonds as the third option once I have topped out my ISA and have got to the point in a 'normal' savings account (currently about £19k, but depends on what rate of tax you pay) where any extra interest will be subject to tax.
tl;dr - even if you put the maximum £50k in premium bonds, your chances of winning 'thousands' (if by which you mean more than about 2 or 3 k a year) are very low. If you've got less than £10k in premium bonds your chances of winning anything at all are low.